Allow a BlockJobTxn to be passed into backup_run, which
will allow the job to join a transactional group if present.

Propagate this new parameter outward into new QMP helper
functions in blockdev.c to allow transaction commands to
pass forward their BlockJobTxn object in a forthcoming patch.

[split up from a patch originally by Stefan and Fam. --js]
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Message-id: 1446765200-3054-12-git-send-email-jsnow@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>