Don't log an internal error when the guest hasn't updated balloon stats

If virDomainMemoryStats is called too soon after domain startup,
QEMU returns:
"error":{"class":"GenericError","desc":"guest hasn't updated any stats yet"}
when we try to query balloon stats.

Check for this reply and log it as OPERATION_INVALID instead of
INTERNAL_ERROR. This means the daemon only logs it at the debug level,
without polluting system logs.

Reported by Laszlo Pal:
https://www.redhat.com/archives/libvirt-users/2014-May/msg00023.html

src/qemu/qemu_monitor_json.c

@@ -1465,12 +1465,22 @@ int qemuMonitorJSONGetMemoryStats(qemuMonitorPtr mon,
                                            NULL)))
         goto cleanup;
 
-    ret = qemuMonitorJSONCommand(mon, cmd, &reply);
+    if ((ret = qemuMonitorJSONCommand(mon, cmd, &reply)) < 0)
+        goto cleanup;
 
-    if (ret == 0)
-        ret = qemuMonitorJSONCheckError(cmd, reply);
+    if ((data = virJSONValueObjectGet(reply, "error"))) {
+        const char *klass = virJSONValueObjectGetString(data, "class");
+        const char *desc = virJSONValueObjectGetString(data, "desc");
 
-    if (ret < 0)
+        if (STREQ_NULLABLE(klass, "GenericError") &&
+            STREQ_NULLABLE(desc, "guest hasn't updated any stats yet")) {
+            virReportError(VIR_ERR_OPERATION_INVALID, "%s",
+                           _("the guest hasn't updated any stats yet"));
+            goto cleanup;
+        }
+    }
+
+    if ((ret = qemuMonitorJSONCheckError(cmd, reply)) < 0)
         goto cleanup;
 
     if (!(data = virJSONValueObjectGet(reply, "return"))) {