nixos/vbox: Move all options to virtualisation.*.

Commit 687caeb renamed services.virtualboxHost to programs.virtualbox,
but according to the discussion on the commit, it's probably a better to
put it into virtualisation.virtualbox instead.

The discussion can be found here:

https://github.com/NixOS/nixpkgs/commit/687caeb#commitcomment-12664978

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/virtualisation/virtualbox-host.nix

@@ -3,7 +3,7 @@
 with lib;
 
 let
-  cfg = config.programs.virtualbox;
+  cfg = config.virtualisation.virtualbox.host;
   virtualbox = config.boot.kernelPackages.virtualbox.override {
     inherit (cfg) enableHardening;
   };
@@ -11,7 +11,7 @@ let
 in
 
 {
-  options.programs.virtualbox = {
+  options.virtualisation.virtualbox.host = {
     enable = mkOption {
       type = types.bool;
       default = false;