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
Remove multipath mapping device descriptor

When detaching a volume from a instance, we need to remove
multipath mapping device descriptor if there are other volumes
mapping to the host instead of do nothing, for some cinder drivers
can not terminate connection for multipath tool sending IOs to
storage array periodically.

Change-Id: I5952b1ced48e1da4cc40b57ee561ec5a2e348545
Closes-bug: #1223975