Various APIs use cgroup to either set or get the statistics of
host or guest. Hotplug or hot unplug new vcpus without creating
or removing the cgroup for the vcpus could cause problems for
those APIs. E.g.

% virsh vcpucount dom
maximum      config        10
maximum      live          10
current      config         1
current      live           1

% virsh setvcpu dom 2

% virsh schedinfo dom --set vcpu_quota=1000
Scheduler      : posix
error: Unable to find vcpu cgroup for rhel6.2(vcpu: 1): No such file or
directory

This patch fixes the problem by creating cgroups for each of the
onlined vcpus, and destroying cgroups for each of the offlined
vcpus.