nixos/virtualbox: Disable hardening for now.

This should display a big fat warning that people can hardly miss until
we have fixed the issues with the host-only-interfaces that persist when
hardining is enabled.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
(cherry picked from commit 5d67b17901ff2c9a18647bd9453c6b0d4294b875)