add lxc.console.logpath

v2: add get_config_item

clear_config_item is not supported, as it isn't for lxc.console, bc
you can do 'lxc.console.logfile =' to clear it.  Likewise save_config
is not needed because the config is now just written through the
unexpanded char*.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>