modules/virtualisation: add unified diskSize opt

See https://github.com/NixOS/nixpkgs/pull/339535 and https://github.com/NixOS/nixpkgs/pull/341058

nixos/modules/virtualisation/qemu-vm.nix

@@ -317,12 +317,11 @@ let
     copyChannel = false;
     OVMF = cfg.efi.OVMF;
   };
-
 in
-
 {
   imports = [
     ../profiles/qemu-guest.nix
+    ./disk-size-option.nix
     (mkRenamedOptionModule
       [
         "virtualisation"
@@ -378,14 +377,6 @@ in
       '';
     };
 
-    virtualisation.diskSize = mkOption {
-      type = types.ints.positive;
-      default = 1024;
-      description = ''
-        The disk size in megabytes of the virtual machine.
-      '';
-    };
-
     virtualisation.diskImage = mkOption {
       type = types.nullOr types.str;
       default = "./${config.system.name}.qcow2";
@@ -1250,6 +1241,8 @@ in
     # override by setting `virtualisation.fileSystems = lib.mkForce { };`.
     fileSystems = lib.mkIf (cfg.fileSystems != { }) (mkVMOverride cfg.fileSystems);
 
+    virtualisation.diskSizeAutoSupported = false;
+
     virtualisation.fileSystems =
       let
         mkSharedDir = tag: share: {