job: Add JOB_STATUS_CHANGE QMP event

This adds a QMP event that is emitted whenever a job transitions from
one status to another.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/tests/qemu-iotests/144 b/tests/qemu-iotests/144
index 00de3c33..4b91571 100755
--- a/tests/qemu-iotests/144
+++ b/tests/qemu-iotests/144
@@ -93,7 +93,7 @@
                                 'arguments': {
                                                 'device': 'virtio0'
                                               }
-                   }" "COMPLETED"
+                   }" '"status": "null"'
 
 echo
 echo === Performing Live Snapshot 2 ===