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

Commit 687caebf 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>