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>
(cherry picked from commit d85fabd68cb0477f7bda4af8a528e27ed7914932)
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>
(cherry picked from commit 8bbf1dc80e92df4374fc97dbc03bfee07bf67ff1)