cg: use one path style throughout cg restore code

This commit is in preparation for the (hopefully last :) restore special cpuset
patch.

Previously, we installed the cgroup service fd after calling
prepare_cgroup_dirs, which meant that we had to carry around the temporary
directory name in order to put things in the right place. The
restore_cgroup_prop function uses the cg service fd instead of carrying around
the full path. This means that we can't sue restore_cgroup_prop, without first
sanitizing the path. Instead, we install the service fd before calling
prepare_cgroup_dirs, and all the code just references that instead of carrying
around the temporary path.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
cg: restore special cpuset props only in cgs criu creates

Previously we were trying to copy the root cg properties recursively to attempt
to correct invalid restores. However, based on some ML discussion, we should
only restore exactly what was dumped. Users need to set up their cg heirarchies
correctly (that is, they should not set up any options in the parent that would
conflict with what was dumped) in order to restore successfully.

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