Factor out setting of allowed devices so that we can update allowed devi...

...ces

Co-Authored-By: Ian Main <imain@redhat.com>
Docker-DCO-1.1-Signed-off-by: Chris Alfonso <calfonso@redhat.com> (github: calfonso)
nit: we're repeating this, can we factor it out into a:

cgroup.GetContainerPath(cgroupHierarchy, parent, name)