um: virtio: disable VQs during suspend

If the system is suspended, the device shouldn't be able to send
anything to it. Disable virtqueues in suspend to simulate this,
and as we might be only using s2idle (kernel services are still
on), prevent sending anything on them as well.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>