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>
cg: restore special cpuset props only in cgs criu creates

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>