Handle missing base images in libvirt block migration.

If a base image cannot be fetched from glance, we must stream it
from the source hypervisor node.

Closes-Bug: 1206887

DocImpact: we now stream base images from the source hypervisor
node if required. This is cool.

Change-Id: I705acc7ca0cd7bec335320041224d1631cb7df95