vcpupin: Fix cpu affinity setting bug of qemu driver

vcpupin: Fix cpu affinity setting bug of qemu driver

There is the case where cpu affinites for vcpu of qemu doesn't work
correctly. For example, if only one vcpupin setting entry is provided
and its setting is not for vcpu0, it doesn't work.

   # virsh dumpxml VM
   ...
   <vcpu>4</vcpu>
   <cputune>
     <vcpupin vcpu='3' cpuset='9-11'/>
   </cputune>
   ...

   # virsh start VM
   Domain VM started

   # virsh vcpuinfo VM
   VCPU:           0
   CPU:            31
   State:          running
   CPU time:       2.5s
   CPU Affinity:   yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy

   VCPU:           1
   CPU:            12
   State:          running
   CPU time:       0.9s
   CPU Affinity:   yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy

   VCPU:           2
   CPU:            30
   State:          running
   CPU time:       1.5s
   CPU Affinity:   yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy

   VCPU:           3
   CPU:            13
   State:          running
   CPU time:       1.7s
   CPU Affinity:   yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy

This patch fixes this problem.

Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>