Especially if the user isn't in the vboxusers group anymore, this gets
VERY noisy, because the VBoxSVC process emits warnings for every single
USB device noting that it's only possible to access it when the user is
in the vboxusers group.
So, we now have a debug attribute, where we can enable it when
necessary.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
