nixos: Give virtualbox-host.nix a better location.

In 14f09e01

, I've introduced the module under modules/programs, because
the legacy virtualbox.nix was also under that path. But because we
already have modules/virtualisation/virtualbox-guest.nix, it really
makes sense to put this module alongside of it as well.

This module thus has no change in functionality and I've tested
evaluation against nixos/tests/virtualbox.nix and the manual.

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