block: Add "drained begin/end" for transactional backup

This ensures the atomicity of the transaction by avoiding processing of
external requests such as those from ioeventfd.

Move the assignment to state->bs up right after bdrv_drained_begin, so
that we can use it in the clean callback. The abort callback will still
check bs->job and state->job, so it's OK.

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>
