qa: allow small allocation diffs for exported rbds

The local filesytem may behave slightly differently. This isn't
foolproof, but seems to be reliable enough on rhel7 rootfs, where
exact comparison was failing.

Fixes: #10002
Signed-off-by: Josh Durgin <jdurgin@redhat.com>
Merge pull request #2899 from jdurgin/wip-import-export-next

qa: allow small allocation diffs for exported rbds

Reviewed-by: Jason Dillaman <dillaman@redhat.com>