nixos: Rename virtualbox to virtualboxGuest.

Especially new users could be confused by this, so we're now marking
services.virtualbox.enable as obsolete and defaulting to
services.virtualboxGuest.enable instead. I believe this now makes it
clear, that this option is for guest additions only.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nixos: Add enable option for programs/virtualbox.

We will simply rename the previous module and add a warning whenever the
module is included directly, pointing the user to the right option and
also enable it as well (in case somebody has missed the option and is
wondering why VirtualBox doesn't work anymore).

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