If a cgroup controller is co-mounted with another, eg

   /sys/fs/cgroup/cpu,cpuacct

Then it is a requirement that there exist symlinks at

   /sys/fs/cgroup/cpu
   /sys/fs/cgroup/cpuacct

pointing to the real mount point. Add support to virCgroupPtr
to detect and track these symlinks
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>