nixos/virtualbox: add warning when for ineffective nixpkgs config

nixpkgs.config.virtualbox.enableExtensionPack doesn't do anything, but
used to. Add a warning for the unsuspecting.

nixos/modules/virtualisation/virtualbox-host.nix

@@ -83,6 +83,8 @@ in
   };
 
   config = mkIf cfg.enable (mkMerge [{
+    warnings = mkIf (config.nixpkgs.config.virtualbox.enableExtensionPack or false)
+      "'nixpkgs.virtualbox.enableExtensionPack' has no effect, please use 'virtualisation.virtualbox.host.enableExtensionPack'";
     boot.kernelModules = [ "vboxdrv" "vboxnetadp" "vboxnetflt" ];
     boot.extraModulePackages = [ kernelModules ];
     environment.systemPackages = [ virtualbox ];