qemu: fix vcpu pinning when not all vcpus are enabled
    
    vcpupin will fail when maxvcpus is larger than current
    vcpu:
    
    virsh vcpupin win7 --vcpu 0 --cpulist 5-6
    error: Requested operation is not valid: cpu affinity is not supported
    
    win7 xml in the command above is like below:
    ...
    <vcpu current="3" placement="static">8</vcpu>
    ...
    
    The reason is vcpu[3] and vcpu[4] have zero tids and should not been
    compared as valid situation in qemuDomainRefreshVcpuInfo().
    
    This issue is introduced by commit 34f7743, which fix recording of vCPU
    pids for MTTCG.
    
    Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
    Reviewed-by: Ján Tomko <jtomko@redhat.com>
    Signed-off-by: Ján Tomko <jtomko@redhat.com>