howto change virtio disk to boot on kvm guest os
create a disk and install os to it by appending “-hda <your_disk_image>" to your virtual machine.
In your guest os, upgrade kernel to 2.6.25 which contains virtio_* drivers. or ubuntu 8.04 also has it.
In guest os, change /boot/grub/device.map from “(hd0) /dev/sda" to “(hd0) /dev/vda"
In guest os, change /boot/grub/menu.list from “root=/dev/sda1" to “root=/dev/vda1“, if you are using UUID, then no need to do this step.
enable para-virtualization by changing “-hda <your_disk_image" to “-drive file=<your_disk_image>,if=virtio,boot=on“