blockjob: introduce block_job_early_fail

blockjob: introduce block_job_early_fail

Outside blockjob.c, block_job_unref is only used when a block job fails
to start, and block_job_ref is not used at all.  The reference counting
thus is pretty well hidden.  Introduce a separate function to be used
by block jobs; because block_job_ref and block_job_unref now become
static, move them earlier in blockjob.c.

Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: John Snow <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Jeff Cody <[email protected]>
Message-id: 20170508141310[email protected]
Signed-off-by: Jeff Cody <[email protected]>