nspawn: suffix the nspawn cgroups with ".nspawn"

nspawn: suffix the nspawn cgroups with ".nspawn"

As discussed with Dan Berrange it's a good idea to suffix all objects in
the cgroup tree with ".something", so that when the system is
partitioned using a resource management tool we can drop objects of
different types into the same partition directory without generate
namespace conflicts.

We'l add this to the Pax Control Group document as soon as write access
to the fdo wiki is restored.