volume mounts of /sys/fs/cgroup results in incorrect behaviour

[mcastelino]

With COR

docker run -it -v /sys/fs/cgroup:/sys/fs/cgroup:ro debian ls -alp /sys/fs/cgroup
total 0
dr-xr-xr-x 2 root root 0 Jun  9 01:20 ./
drwxr-xr-x 6 root root 0 Jun  9 01:20 ../

With runc

total 0
drwxr-xr-x 13 root root 340 Jun  9 00:44 ./
drwxr-xr-x 10 root root   0 Jun  9 00:51 ../
dr-xr-xr-x  6 root root   0 Jun  9 00:51 blkio/
lrwxrwxrwx  1 root root  11 Jun  9 00:44 cpu -> cpu,cpuacct
dr-xr-xr-x  6 root root   0 Jun  9 00:51 cpu,cpuacct/
lrwxrwxrwx  1 root root  11 Jun  9 00:44 cpuacct -> cpu,cpuacct
dr-xr-xr-x  3 root root   0 Jun  9 00:51 cpuset/
dr-xr-xr-x  6 root root   0 Jun  9 00:51 devices/
dr-xr-xr-x  3 root root   0 Jun  9 00:51 freezer/
dr-xr-xr-x  3 root root   0 Jun  9 00:51 hugetlb/
dr-xr-xr-x  6 root root   0 Jun  9 00:51 memory/
lrwxrwxrwx  1 root root  16 Jun  9 00:44 net_cls -> net_cls,net_prio
dr-xr-xr-x  3 root root   0 Jun  9 00:51 net_cls,net_prio/
lrwxrwxrwx  1 root root  16 Jun  9 00:44 net_prio -> net_cls,net_prio
dr-xr-xr-x  3 root root   0 Jun  9 00:51 perf_event/
dr-xr-xr-x  6 root root   0 Jun  9 00:51 pids/
dr-xr-xr-x  6 root root   0 Jun  9 00:51 systemd/

We need to support smarter handling of these special cases system volumes.
This is needed if we want to support system/machine containers

These cannot be exposed via 9p. However these volumes can provide hints on how we perform the mount space setup within the Clear Containers VM.