qa/workunits/rbd: yet another attempt to improve rbd-nbd unmap

qa/workunits/rbd: yet another attempt to improve rbd-nbd unmap

Previously it still could race when unmap_device returned success
because the device was not found in `rbd-nbd list-mapped` (the nbd
device was removed) but the test failed because the process was still
found in the ps table.

Fixes: https://tracker.ceph.com/issues/47394
Signed-off-by: Mykola Golub <mgolub@suse.com>
(cherry picked from commit f0c69761c8036a57319ead5cdf97cebb0f0fb5d7)

Conflicts:
	qa/workunits/rbd/rbd-nbd.sh
- omit changes in tests that are not in nautilus