blockjob: Introduce reference count and fix reference to job->bs

blockjob: Introduce reference count and fix reference to job->bs

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 <[email protected]>
Signed-off-by: John Snow <[email protected]>
Message-id: 1446765200[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>