Fix lua telnet vlm_message_to_string

Fix lua telnet vlm_message_to_string

In the current lua vlm implementation, messages can have value xor
children but not both. But the "show media" command gives media a
value : ( %d broadcast - %d vod ) and children (if any). (cf
src/input/vlmshell.c:1502)

Signed-off-by: Rémi Duraffort <ivoire@videolan.org>