Manually defining grub "BIOS" and OS mapping for VirtIO devices.

Grub is unable to create a BIOS to OS device mapping for VirtIO devices
such as /dev/vda. The device-map file helps grub to find the correct
mapping. This fixes #3

For more informations, see
http://www.linux-kvm.org/page/Boot_from_virtio_block_device#Steps
http://www.gnu.org/software/grub/manual/legacy/Device-map.html