The most recent release of the open source virtio drivers released now adds support for windows 8 guests. These drivers are available in iso format that you mount in your KVM guest operating system. Windows 8 is not officially supported in upstream qemu yet but you can run it as of qemu 1.3 using an experimental new machine type called q35. You can download these latest drivers at the following link. The link has also been updated on the front page here at www.linux-kvm.com.
I have not tested these drivers yet. Please post any comments or questions if you get around to testing them.