cgroup: allow restoring from multi-headed freezer cgroups

b428a3a2 allows dumping containers with
multi-headed freezer cgroups, but we can't restore these containers without
some help at restore time too.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>