And add a testcase to catch regressions.

Without this patch, restoring a snapshot of an overlayfs based
container fails, because we do not pass in LXC_CLONE_SNAPSHOT,
and overlayfs does not support clone without snapshot.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>