virtualisation: Make parallels handling more lazy

Moves the condition to allow parallels as an unfree module up one level.
This is required for users who don't instantiate their own nixpkgs
instance which causes `options.nixpkgs.pkgs.isDefined` being false.
In this case, although `cfg.parallels.enable` is false, the
`allowUnfreePredicate` will still be set resulting an error complaining
about `allowUnfreePredicate` option being used but not defined.

Author: britter

modules/nixos/virtualisation.nix

@@ -97,10 +97,8 @@ in
 
     # parallels
     hardware.parallels.enable = lib.mkIf cfg.parallels.enable (lib.mkDefault true);
-    nixpkgs.config = lib.mkIf (!options.nixpkgs.pkgs.isDefined) {
-      allowUnfreePredicate = lib.mkIf cfg.parallels.enable (
-        pkg: builtins.elem (lib.getName pkg) [ "prl-tools" ]
-      );
+    nixpkgs.config = lib.mkIf (!options.nixpkgs.pkgs.isDefined && cfg.parallels.enable) {
+      allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [ "prl-tools" ];
     };
   };
 }