python3: Don't fail in list_containers on ValueError

python3: Don't fail in list_containers on ValueError

ValueError typically means that the user doesn't have permissions to
access the directory. Raising an exception there isn't consistent with
other error behaviour of list_containers which simple returns an empty
tuple.

So simply catch the exception and ignore it. An error message is already
printed by LXC itself anyway.

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