cgroup: Change virCgroupRemove to remove all descendant groups at first

cgroup: Change virCgroupRemove to remove all descendant groups at first

As same as normal directories, a cgroup cannot be removed if it
contains sub groups. This patch changes virCgroupRemove to remove
all descendant groups (subdirectories) of a target group before
removing the target group.

The handling is required when we run lxc with ns subsystem of cgroup.
Ns subsystem automatically creates child cgroups on every process
forks, but unfortunately the groups are not removed on process exits,
so we have to remove them by ourselves.

With this patch, such child (and descendant) groups are surely removed
at lxc shutdown, i.e., lxcVmCleanup which calls virCgroupRemove.