lxclock: fall back to /tmp if xdg_rundir is not writeable

This happens for instance if you run a test under sudo which
then runs lxc commands under 'su - <someuser>'

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