[CRIU] cgroup fixes

[Tycho Andersen]

Hi guys,

Here's a set of fixes for the cgroup04 test. These cause it to fail without
Andrey's patch here:

https://lists.openvz.org/pipermail/criu/2016-June/029367.html

so we should take that one first.

Also, this points out a wider bug: we don't dump criu's cgset, so we don't
preserve empty cgroups that are children of cgroups in criu's set. I know we
have avoided dumping criu's cg set before, so perhaps we want to leave it this
way, which is why I've only sent a patch noting it in the comment. I can write
a patch to fix it if we want, but I think it's a small bug, if that :)

Tycho