Fixes error on live-migration of volume-backed vm

Live-migrating a volume-backed vm (i.e. booted from volume)
is currently broken.

This patch fixes the case where a volume-backed vm is to be
live-migrated without shared storage on compute nodes or
ephemeral volumes attached to the instance. Specifically, it
stops create_images_and_backing() from blowing up when no disk
info is supplied.

Change-Id: Icec7a6e7225ebe029e24d3be303c9ab01818f30e
Fixes: bug 1235182
(cherry picked from commit 0cbb231cd14c8cb767b67d89b14d0ef46b3e8018)