nixos/virtualbox: Fix warning on enableHardening.

The warning was displayed whenever services.virtualboxHost.enable was
true, but if people were to enable hardening, they'd still get that
annoying message.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
virtualbox: Fix extension pack without hardening.

We divert to the $out/share/virtualbox directory only if we have
hardening enabled, so let's put the extension pack into
$out/libexec/virtualbox instead if we're compiling without hardening.

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