Re-arrange the code so that we only grab the container object when doing
something more than building a simple list of existing containers.

This means that now the following calls can run unprivileged:
 - lxc-ls
 - lxc-ls -1

Everything else will still require root privileges.
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>