qapi: fix leak in unit tests

qmp_output_get_qobject() increments the qobject's reference count. Since
we currently pass this straight into qobject_to_json() so we can feed
the data into a QMP input visitor, we never actually free the underlying
qobject when qmp_output_visitor_cleanup() is called. This causes leaks
on all of the QMP serialization tests.

Fix this by holding a pointer to the qobject and decref'ing it before
returning from qmp_deserialize().

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

tests/test-visitor-serialization.c

@@ -657,11 +657,16 @@ static void qmp_deserialize(void **native_out, void *datap,
                             VisitorFunc visit, Error **errp)
 {
     QmpSerializeData *d = datap;
-    QString *output_json = qobject_to_json(qmp_output_get_qobject(d->qov));
-    QObject *obj = qobject_from_json(qstring_get_str(output_json));
+    QString *output_json;
+    QObject *obj_orig, *obj;
+
+    obj_orig = qmp_output_get_qobject(d->qov);
+    output_json = qobject_to_json(obj_orig);
+    obj = qobject_from_json(qstring_get_str(output_json));
 
     QDECREF(output_json);
     d->qiv = qmp_input_visitor_new(obj);
+    qobject_decref(obj_orig);
     qobject_decref(obj);
     visit(qmp_input_get_visitor(d->qiv), native_out, errp);
 }