qemu: monitor: Don't leak @props with non-JSON in qemuMonitorAddObject

The function comment states that @props is always consumed, even on
failure. This was not true with the failure if the monitor is not using
QMP.

src/qemu/qemu_monitor.c

@@ -3246,11 +3246,13 @@ qemuMonitorAddObject(qemuMonitorPtr mon,
               mon, type, objalias, props);
     int ret = -1;
 
-    if (mon->json)
+    if (mon->json) {
         ret = qemuMonitorJSONAddObject(mon, type, objalias, props);
-    else
+    } else {
+        virJSONValueFree(props);
         virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
                        _("object adding requires JSON monitor"));
+    }
 
     return ret;
 }