qemu: cgroup: Properly set up vcpu pinning

qemu: cgroup: Properly set up vcpu pinning

When the default cpuset or automatic numa placement is used libvirt
would place the whole parent cgroup in the specified cpuset. This then
disallowed to re-pin the vcpus to a different cpu.

This patch pins only the vcpu threads to the default cpuset and thus
allows to re-pin them later.

The following config would fail to start:
<domain type='kvm'>
  ...
  <vcpu placement='static' cpuset='0-1' current='2'>4</vcpu>
  <cputune>
    <vcpupin vcpu='0' cpuset='2-3'/>
    ...

This is a regression since a39f69d2b.