cgroup2: implement runc ps

Implemented `runc ps` for cgroup v2 , using a newly added method `m.GetUnifiedPath()`.
Unlike the v1  implementation that checks `m.GetPaths()["devices"]`, the v2 implementation does not require the device controller to be available.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

libcontainer/container_linux_test.go

@@ -54,6 +54,10 @@ func (m *mockCgroupManager) GetPaths() map[string]string {
 	return m.paths
 }
 
+func (m *mockCgroupManager) GetUnifiedPath() (string, error) {
+	return "", fmt.Errorf("unimplemented")
+}
+
 func (m *mockCgroupManager) Freeze(state configs.FreezerState) error {
 	return nil
 }