Must use UUIDs rather than /dev/xvda1 when regenerating grub

When rebooting under xen the device won't be the same as the original device
if we're running in the cloud, as the original device will be xvda1 (or similar)
but the new device will be sda1
Merge pull request #55 from BobBall/uuids

Must use UUIDs rather than /dev/xvda1 when regenerating grub