nixos/virtualbox/hostonlyif: Fix writing to /root.

Creates unnecessary cruft in the root users home directory, which we
really don't need. Except the log, but therefore we now cat the log to
stderr and the private temporary directory is cleaned up afterwards.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nixos/tests/virtualbox: Improve logging.

This also makes showvminfo obsolete, as we get the same information from
the hosts log.

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