nixos/qemu-vm: convert tmpfiles rules to settings

This change replaces the previously hard-coded `/boot` path with a
reference to `efiSysMountPoint` and more importantly this change makes
it possible to override these rules in scenarios in which they are not
desired.

One such scenario would be when `systemd-gpt-auto-generator(8)` is used
to automount the ESP. Consider this section from the mentioned manpage:

> The ESP is mounted to /boot/ if that directory exists and is not used
> for XBOOTLDR, and otherwise to /efi/. Same as for /boot/, an automount
> unit is used. The mount point will be created if necessary.

Prior to this change, the ESP would be automounted under `/efi` on first
boot, then the previous tmpfiles rules caused `/boot` to be created.
Following the quote above, this meant that the ESP is mounted under
`/boot` for each subsequent boot.

nixos/modules/virtualisation/qemu-vm.nix

@@ -1066,10 +1066,18 @@ in
         ''}
       '';
 
-    systemd.tmpfiles.rules = lib.mkIf config.boot.initrd.systemd.enable [
-      "f /etc/NIXOS 0644 root root -"
-      "d /boot 0644 root root -"
-    ];
+    systemd.tmpfiles.settings."10-qemu-vm" = lib.mkIf config.boot.initrd.systemd.enable {
+      "/etc/NIXOS".f = {
+        mode = "0644";
+        user = "root";
+        group = "root";
+      };
+      "${config.boot.loader.efi.efiSysMountPoint}".d = {
+        mode = "0644";
+        user = "root";
+        group = "root";
+      };
+    };
 
     # After booting, register the closure of the paths in
     # `virtualisation.additionalPaths' in the Nix database in the VM.  This