blockjob: reorganize block_job_completed_txn_abort

This splits the part that touches job states from the part that invokes
callbacks.  It will make the code simpler to understand once job states will
be protected by a different mutex than the AioContext lock.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20170508141310.8674-11-pbonzini@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
