[libvirt] pvspinlock not accepted by libvirt

Hello, I have been experimenting with the various options to squeeze out the last bit of performance gains so I added the <pvspinlock/> feature to my machines. However when saving the xml, libvirt complains and refuses to accept this tag. AFAIK this is a feature that can be used by KVM too. My guests are Linux of course. So what am I doing wrong here?