conf: Ignore vcpupin for not onlined vcpus when parsing

conf: Ignore vcpupin for not onlined vcpus when parsing

Setting pinning policy for vcpu which exceeds current vcpus number
just makes no sense, however, it could cause various problems, E.g.

<vcpu current='1'>4</vcpu>
<cputune>
  <vcpupin vcpuid='3' cpuset='4'/>
</cputune>

% virsh start linux
error: Failed to start domain linux
error: cannot set CPU affinity on process 32534: No such process

We must have some odd codes underlying which produces the
"on process 32534", but the point is why we not to prevent
earlier when parsing? Note that this is only one of the
problem it could cause.

This patch is to ignore the <vcpupin> for not onlined vcpus.