tests/virtualbox: Start systemwide DBus in guests.

We want to check whether DBus functionality is working, so let's make
sure it is running in our mini-initrd.

DBus unfortunately requires to have users properly set up and another
configuration file other than in ${dbus.daemon}/etc/dbus-1/system.conf,
so we do provide that as well.

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