stage1: support hybrid cgroup hierarchy
stage1: support hybrid cgroup hierarchy

systemd introduced the hybrid cgroup hierarchy that mounts a cgroup v1
and v2 hierarchy in parallel. After systemd v233 the cgroup v2 hierarchy
is mounted in `/sys/fs/cgroup/unified`. This breaks rkt (specifically
flavor host) because we mount `/sys/fs/cgroup` ourselves and were not
creating the `unified` directory.

Let's create `/sys/fs/cgroup/unified` in CreateCgroup() to support the
hybrid cgroup hierarchy. If the system or stage1 doesn't use the hybrid
hierarchy, having this directory created doesn't hurt.

lucasallan/rkt,joshix/rkt,coreos/rkt,rkt/rkt,lucasallan/rkt,rkt/rkt,lucasallan/rkt,kinvolk/rkt,Shashwatsh/rkt,alban/rkt,kinvolk/rkt,ybubnov/rkt,kinvolk/rkt,rkt/rkt,coreos/rkt,joshix/rkt,joshix/rkt,lucasallan/rkt,rkt/rkt,kinvolk/rkt,ybubnov/rkt,alban/rkt,coreos/rkt,Shashwatsh/rkt,joshix/rkt,alban/rkt,ybubnov/rkt,Shashwatsh/rkt