Linux KVM virtio drivers are included in FreeBSD 8.4. All I had to do was enable them in the kernel hints file and recompile/build the image.
I added the following lines to the end of M0N0WALL_GENERIC.i386
#virtio drivers for linux kvm
device virtio
device virtio_pci
device virtio_balloon
device virtio_blk
device virtio_scsi
device vtnet
Nothing else was changed.