lxc_stop: return success if api_shutdown succeeded

I originally forgot to set ret = 0 if it succeeded, meaning that a
simple 'lxc-stop -n container1' returns failure even though the
stop succeeded.

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