vnc: handle case where tag vncport in invalid

            raise cmdError("No selected object")

        for vm in ans["objects"]:

            host_id = vm["p"]

            try:

                dest_port = int(vm["vncport"])

            except ValueError:

                raise cmdError('Invalid value for tag vncport')

            # check if we have the right to forward on this hypervisor

            ans2 = self.rpc.call("list", "id:%s" % host_id, method="forward")

            if len(ans2["objects"]) == 0: