Ensure image property not set to None in build_request_spec()

This is in preparation for having scheduler.util.build_request_spec()
construct the request_spec in the live migration code. Specifically, this
change is needed to avoid reintroducing bug #1199811.

Change-Id: I0cc6b6ec7be7a0afbe184b208f2e16fd5cd4452c
Related-Bug: #1224014