Removing a quorum child node with x-blockdev-change results in a quorum
driver state that cannot be recreated with create options because it
would require a list with gaps. This causes trouble in at least
.bdrv_refresh_filename().

Document this problem so that we won't accidentally mark the command
stable without having addressed it.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>