tests/virtualbox: Add a subtest for host USB.

Unfortunately, we can't test whether USB is really working, but we can
make sure that VirtualBox has access to the USB devices.

This is essentially testing #9736, which I haven't yet been able to
reproduce though, but it makes sense to test it so it won't happen in
future releases.

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