lxc-ls: Add lxc.start.auto and lxc.group support

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
lxc-ls: Add workaround for nested containers

When accessing nested containers, hardcode the path to their config.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>