lxc: cgfs: handle lxcfs

lxc: cgfs: handle lxcfs

When containers have lxcfs mounted instead of cgroupfs, we have to
process /proc/self/mountinfo a bit differently.  In particular, we
should look for fuse.lxcfs fstype, we need to look elsewhere for the
list of comounted controllers, and the mount_prefix is not a cgroup path
which was bind mounted, so we should ignore it, and named subsystems
show up without the 'name=' prefix.

With this patchset I can start containers inside a privileged lxd
container with lxcfs mounted (i.e. without cgroup namespaces).

Closes #830

Signed-off-by: Ubuntu <ubuntu@localhost.localdomain>