Comment 6 for bug 367137

Mark, to answer your above question from yesterday, you should be able to change the number of cores by editing the xml file that describes the guest. It should be located at /etc/libvirt/qemu/<name>.xml
There should be a line like
   <vcpu>1</vcpu>
where you can change the number of CPUs the guest sees. Make your change and save, then either redefine the xml file (be sure not to undefine!)
   sudo virsh
      # define /etc/libvirt/qemu/<name>.xml
      # quit
substitute it the correct xml of course. Alternatively you should be able to just restart libvirtd, though I'm not sure what effect this may have on running guests:
   sudo /etc/init.d/libvirt-bin restart

The above has worked for me in the past, I hope it helps.
Thanks for finding the Red Hat bug, I'll attach it to this report so we can track it.