lxc-init used to be under /usr/lib/lxc.  Now it is under
/usr/lib/<multiarch>/lxc, but old containers will still have it under
/usr/lib/lxc.  So search for a valid lxc-init to run.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>