qemuMonitorAddObject: Fix semantics of @alias

The callers of qemuMonitorAddObject rely on the fact that @alias is
filled only when the object is added successfully. This is documented
but the code didn't behave like that.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_monitor.c

@@ -3078,11 +3078,14 @@ qemuMonitorAddObject(qemuMonitorPtr mon,
     if (alias)
         tmp = g_strdup(id);
 
-    ret = qemuMonitorJSONAddObject(mon, props);
+    if (qemuMonitorJSONAddObject(mon, props) < 0)
+        goto cleanup;
 
     if (alias)
         *alias = g_steal_pointer(&tmp);
 
+    ret = 0;
+
  cleanup:
     VIR_FREE(tmp);
     virJSONValueFree(*props);