export lxc_cmd_get_config_item via API and use in lxc-info

This allows external users to query network related config items
from the running containers.

changes since v1:
- function name change

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>