show: Don't continue showing messages with the descriptor of nested one

When we dive into showing nested message, the call to it will
overwrite the ctl->Arg field with the descriptor of the nested
one. As a result, after the call returns we continue showing
old message with new descriptor.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>