virtualisation/qemu-vm: disallow nullable diskSize

Because `virtualisation.diskSize = null` does result in a broken vm runner,
see https://github.com/NixOS/nixpkgs/issues/292901.

diskSize was declared to be nullable when it first got types in a
tree-wide commit:
https://github.com/NixOS/nixpkgs/commit/30f0faac22b6d8a6c6b308a0aa44ee672f2827eb

But it seemingly never actually supported it, as "${cfg.diskSize}M" is
passed to qemu-img create, which doesn't allow an empty size parameter.

closes: https://github.com/NixOS/nixpkgs/issues/292901

nixos/modules/virtualisation/qemu-vm.nix

@@ -329,7 +329,7 @@ in
 
     virtualisation.diskSize =
       mkOption {
-        type = types.nullOr types.ints.positive;
+        type = types.ints.positive;
         default = 1024;
         description = ''
             The disk size in megabytes of the virtual machine.