Comment 17 for bug 604335

Appears there are four different udev disk directories not three.

/dev/disk/by-id

/dev/disk/by-label

/dev/disk/by-path

/dev/disk/by-uuid

If you have only virtio disk (/dev/vda) then you will not have /dev/disk/by-id

If you have only virtio disk (/dev/vda) plus a disk label you will have /dev/disk/by-label and that is good enough for grub-pc wrt. installing/updating.