block/dirty-bitmaps: prohibit readonly bitmaps for backups
drive and blockdev backup cannot use readonly bitmaps, because the
sync=incremental mechanism actually edits the bitmaps on success.
If you really want to do this operation, use a copied bitmap.
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id:
20190301191545.8728-5-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>