libvirt users - Dec 2011 - Can I configure cores instead of CPU's

Hi All,

Scientific Linux 6.1 x64
qemu-kvm-0.12.1.2-2.160.el6_1.2.x86_64

My XP-Pro guest will only let me use two CPUs.
Is there a way I can tell Virt-Manager to use
one CPU with six cores instead of four separate
CPUs?

I can manually insert things with
          virsh edit KVM-WinXP

  Currently, I have:
          <vcpu>6</vcpu>

I found an example of a run string:

     -smp 2,sockets=2,cores=1,threads=1

But I will be darned if I can figure out the virsh xml syntax.

Many thanks,
-T