Add a priority level for overrides in VM tests

Now that overriding fileSystems in qemu-vm.nix works again, it's
important that the VM tests that add additional file systems use the
same override priority.  Instead of using the same magic constant
everywhere, they can now use mkVMOverride.

http://hydra.nixos.org/build/6695561

lib/modules.nix

@@ -284,9 +284,10 @@ rec {
       inherit priority content;
     };
 
-  mkOptionDefault = mkOverride 1001;
-  mkDefault = mkOverride 1000;
+  mkOptionDefault = mkOverride 1001; # priority of option defaults
+  mkDefault = mkOverride 1000; # used in config sections of non-user modules to set a default
   mkForce = mkOverride 50;
+  mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’
 
   mkFixStrictness = id; # obsolete, no-op