treewide: automatically md-convert option descriptions

the conversion procedure is simple:

 - find all things that look like options, ie calls to either `mkOption`
   or `lib.mkOption` that take an attrset. remember the attrset as the
   option
 - for all options, find a `description` attribute who's value is not a
   call to `mdDoc` or `lib.mdDoc`
 - textually convert the entire value of the attribute to MD with a few
   simple regexes (the set from mdize-module.sh)
 - if the change produced a change in the manual output, discard
 - if the change kept the manual unchanged, add some text to the
   description to make sure we've actually found an option. if the
   manual changes this time, keep the converted description

this procedure converts 80% of nixos options to markdown. around 2000
options remain to be inspected, but most of those fail the "does not
change the manual output check": currently the MD conversion process
does not faithfully convert docbook tags like <code> and <package>, so
any option using such tags will not be converted at all.

nixos/modules/virtualisation/libvirtd.nix

@@ -29,7 +29,7 @@ let
       enable = mkOption {
         type = types.bool;
         default = true;
-        description = ''
+        description = lib.mdDoc ''
           Allows libvirtd to take advantage of OVMF when creating new
           QEMU VMs with UEFI boot.
         '';
@@ -47,7 +47,7 @@ let
         default = [ pkgs.OVMF.fd ];
         defaultText = literalExpression "[ pkgs.OVMF.fd ]";
         example = literalExpression "[ pkgs.OVMFFull.fd pkgs.pkgsCross.aarch64-multiplatform.OVMF.fd ]";
-        description = ''
+        description = lib.mdDoc ''
           List of OVMF packages to use. Each listed package must contain files names FV/OVMF_CODE.fd and FV/OVMF_VARS.fd or FV/AAVMF_CODE.fd and FV/AAVMF_VARS.fd
         '';
       };
@@ -59,7 +59,7 @@ let
       enable = mkOption {
         type = types.bool;
         default = false;
-        description = ''
+        description = lib.mdDoc ''
           Allows libvirtd to use swtpm to create an emulated TPM.
         '';
       };
@@ -68,7 +68,7 @@ let
         type = types.package;
         default = pkgs.swtpm;
         defaultText = literalExpression "pkgs.swtpm";
-        description = ''
+        description = lib.mdDoc ''
           swtpm package to use.
         '';
       };
@@ -91,7 +91,7 @@ let
       runAsRoot = mkOption {
         type = types.bool;
         default = true;
-        description = ''
+        description = lib.mdDoc ''
           If true,  libvirtd runs qemu as root.
           If false, libvirtd runs qemu as unprivileged user qemu-libvirtd.
           Changing this option to false may cause file permission issues
@@ -105,7 +105,7 @@ let
         default = ''
           namespaces = []
         '';
-        description = ''
+        description = lib.mdDoc ''
           Contents written to the qemu configuration file, qemu.conf.
           Make sure to include a proper namespace configuration when
           supplying custom configuration.
@@ -115,7 +115,7 @@ let
       ovmf = mkOption {
         type = ovmfModule;
         default = { };
-        description = ''
+        description = lib.mdDoc ''
           QEMU's OVMF options.
         '';
       };
@@ -123,7 +123,7 @@ let
       swtpm = mkOption {
         type = swtpmModule;
         default = { };
-        description = ''
+        description = lib.mdDoc ''
           QEMU's swtpm options.
         '';
       };
@@ -162,11 +162,11 @@ in
     enable = mkOption {
       type = types.bool;
       default = false;
-      description = ''
+      description = lib.mdDoc ''
         This option enables libvirtd, a daemon that manages
         virtual machines. Users in the "libvirtd" group can interact with
         the daemon (e.g. to start or stop VMs) using the
-        <command>virsh</command> command line tool, among others.
+        {command}`virsh` command line tool, among others.
       '';
     };
 
@@ -174,7 +174,7 @@ in
       type = types.package;
       default = pkgs.libvirt;
       defaultText = literalExpression "pkgs.libvirt";
-      description = ''
+      description = lib.mdDoc ''
         libvirt package to use.
       '';
     };
@@ -182,7 +182,7 @@ in
     extraConfig = mkOption {
       type = types.lines;
       default = "";
-      description = ''
+      description = lib.mdDoc ''
         Extra contents appended to the libvirtd configuration file,
         libvirtd.conf.
       '';
@@ -192,7 +192,7 @@ in
       type = types.listOf types.str;
       default = [ ];
       example = [ "--verbose" ];
-      description = ''
+      description = lib.mdDoc ''
         Extra command line arguments passed to libvirtd on startup.
       '';
     };
@@ -200,7 +200,7 @@ in
     onBoot = mkOption {
       type = types.enum [ "start" "ignore" ];
       default = "start";
-      description = ''
+      description = lib.mdDoc ''
         Specifies the action to be done to / on the guests when the host boots.
         The "start" option starts all guests that were running prior to shutdown
         regardless of their autostart settings. The "ignore" option will not
@@ -212,7 +212,7 @@ in
     onShutdown = mkOption {
       type = types.enum [ "shutdown" "suspend" ];
       default = "suspend";
-      description = ''
+      description = lib.mdDoc ''
         When shutting down / restarting the host what method should
         be used to gracefully halt the guests. Setting to "shutdown"
         will cause an ACPI shutdown of each guest. "suspend" will
@@ -223,7 +223,7 @@ in
     allowedBridges = mkOption {
       type = types.listOf types.str;
       default = [ "virbr0" ];
-      description = ''
+      description = lib.mdDoc ''
         List of bridge devices that can be used by qemu:///session
       '';
     };
@@ -231,7 +231,7 @@ in
     qemu = mkOption {
       type = qemuModule;
       default = { };
-      description = ''
+      description = lib.mdDoc ''
         QEMU related options.
       '';
     };