Use the cgroup name for cpuset for lxc-ps

On my Ubuntu 13.10 system, lxc-ps was always giving empty output. The
output of /proc/$initpid/cgroup was

11:name=systemd:/user/1000.user/c3.session
10:hugetlb:/container
9:perf_event:/container
8:blkio:/container
7:freezer:/container
6:devices:/container
5:memory:/container
4:cpuacct:/container
3:cpu:/container
2:cpuset:/container

Using the cpuset line should be a safer option.

Signed-off-by: Nick Huber <nicholashuber@gmail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>