Please add use support ( use flag ) virtio-serial feature in app-emulation/qemu-kvm and app-emulation/qemu for more info see http://www.linux-kvm.org/page/VMchannel_Requirements
You just need a recent version of qemu / qemu-kvm and it ought to Just Work(TM). I would suggest updating to the newest stable version, 0.13.
*qemu-kvm-0.13.0 (28 Oct 2010) 28 Oct 2010; Luca Barbato <lu_zero@gentoo.org> +qemu-kvm-0.13.0.ebuild: New version, user targets support improved Tnx, lu_zero. Fixed