virtualisation/kubevirt: use mkImageMediaOverride...

...for filesystem  options. Before this change,
users would typically encounter conflicting option definitions
when trying to build an image for a generic nixos closure, i.e.
`nixos-rebuild build-image --image-variant kubevirt --flake .#my-host`

nixos/modules/virtualisation/kubevirt.nix

@@ -12,7 +12,7 @@
   ];
 
   config = {
-    fileSystems."/" = {
+    fileSystems."/" = lib.mkImageMediaOverride {
       device = "/dev/disk/by-label/nixos";
       fsType = "ext4";
       autoResize = true;