block: Add "drained begin/end" for transactional blockdev-backup Similar to the previous patch, make sure that external events are not dispatched during transaction operations. Signed-off-by: Fam Zheng <famz@redhat.com> Reviewed-by: Jeff Cody <jcody@redhat.com> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>