[CRIU] Restore into different cgroups

[Pavel Emelyanov]

Serge, Tycho,

Some time ago we've discussed that it would be good to have an
ability to dump a container sitting in cgroups e.g. /memory/lxc/0/
but to restore one into some different path e.g. /memory/foo/lxc/0/.

We're having similar issue in OpenVZ -- while migrating the container
we can request for its ID change and thus we will need to change
cgroup paths e.g. from /memory/vz/100 into /memory/vz/101.

I've noticed that currently all the paths in the cgroup image contains
only absolute paths which makes cgroup dir (actually root) change
quite tricky.

While I'm thinking what can be done about it, maybe you have already
researched this and have some idea ready?

Thanks,
Pavel