QMP: Only handle converted commands

Looks like I dropped this check when addressing the 'query-'
commands request.

QMP should only handle converted commands, obviously.

Reported-by: Markus Armbruster <armbru@redhat.com>

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

monitor.c

@@ -4070,7 +4070,7 @@ static void handle_qmp_command(JSONMessageParser *parser, QList *tokens)
                       qobject_from_jsonf("{ 'item': %s }", info_item));
     } else {
         cmd = monitor_find_command(cmd_name);
-        if (!cmd) {
+        if (!cmd || !monitor_handler_ported(cmd)) {
             qemu_error_new(QERR_COMMAND_NOT_FOUND, cmd_name);
             goto err_input;
         }