virtualisation/virtualbox-image: use mkImageMediaOverride...

...for filesystem and 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 virtualbox --flake .#my-host`

nixos/modules/virtualisation/virtualbox-image.nix

@@ -275,7 +275,7 @@ in
     };
 
     fileSystems =
-      {
+      lib.mkImageMediaOverride {
         "/" = {
           device = "/dev/disk/by-label/nixos";
           autoResize = true;