um: virtio_uml: mark device as unregistered when breaking it

Mark the device as not registered anymore when scheduling the work to
remove it. Otherwise we could end up scheduling the work multiple times
in a row, including scheduling it while it is already running.

Fixes: af9fb41e

 ("um: virtio_uml: Fix broken device handling in time-travel")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>