um: virtio_uml: move device breaking into workqueue

We should not be calling virtio_break_device from an IRQ context.
Move breaking the device into the workqueue so that it is done from
a reasonable context.

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>