block: Add "drained begin/end" for transactional external snapshot This ensures the atomicity of the transaction by avoiding processing of external requests such as those from ioeventfd. 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>