New cgroups API for atomically creating machine cgroups

New cgroups API for atomically creating machine cgroups

Instead of requiring one API call to create a cgroup and
another to add a task to it, introduce a new API
virCgroupNewMachine which does both jobs at once. This
will facilitate the later code to talk to systemd to
achieve this job which is also atomic.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>