Pass None to image if booted from volume in live migration

The destination check in live migration will fetch the image
information from glance and it will throw ImageNotFound if
the instance is booted from volume since there is no image id
on the instance

Fix bug 1170596

Conflicts:

	nova/scheduler/driver.py

Change-Id: Ie683a3ca5d6430c52ead77b41f98fbcb4114ea1e