qobject: let object_property_get_str() use new API

We can simplify object_property_get_str() using the new
qobject_get_try_str().
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-5-peterx@redhat.com>
[eblake: rebase context of qobject_to() macro]
Signed-off-by: NEric Blake <eblake@redhat.com>

qom/object.c

@@ -1136,18 +1136,15 @@ char *object_property_get_str(Object *obj, const char *name,
                               Error **errp)
 {
     QObject *ret = object_property_get_qobject(obj, name, errp);
-    QString *qstring;
     char *retval;
 
     if (!ret) {
         return NULL;
     }
-    qstring = qobject_to(QString, ret);
-    if (!qstring) {
+
+    retval = g_strdup(qobject_get_try_str(ret));
+    if (!retval) {
         error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name, "string");
-        retval = NULL;
-    } else {
-        retval = g_strdup(qstring_get_str(qstring));
     }
 
     qobject_decref(ret);