Add reference count to block job, meanwhile move the ownership of the
reference to job->bs from the caller (which is released in two
completion callbacks) to the block job itself. It is necessary for
block_job_complete_sync to work, because block job shouldn't live longer
than its bs, as asserted in bdrv_delete.

Now block_job_complete_sync can be simplified.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1446765200-3054-6-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>