librbd: gracefully handle deleted/renamed pools

snap_unprotect and list_children both attempt to scan all
pools.  If a pool is deleted or renamed during the scan,
the methods would previously return -ENOENT.  Both methods
have been modified to more gracefully handle this condition.

Fixes: #10270
Backport: giant, firefly, dumpling
Signed-off-by: Jason Dillaman <dillaman@redhat.com>