Grub with virtio disk

When coding around with the pentoo-installer in virtual machines, grub simply refused to detect the virtual device mounted on /dev/vda.

The solution was in this article:
https://wiki.archlinux.org/index.php/KVM#Preparing_an_.28Arch.29_Linux_guest

# grub
> device (hd0) /dev/vda
> root (hd0,0)
> setup (hd0)
> quit

Also put this in /boot/grub/device.map:

(hd0) /dev/vda