Use UUIDs in /boot/grub/grub.cfg for VM builds rather than hardcoding /dev/sda

This allows booting the image in KVM as either sda or vda without
requiring any manual changes in /boot/grub/grub.cfg.  Use
FIXED_DISK_IDENTIFIERS="yes" for VM builds by default.

Patch rebased by Michael Prokop <mika@grml.org>
Fixes #29 @ github