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
Make LiveMigrateTask use build_request_spec()

Some filters like TrustedFilter makes use of extra_specs. Currently,
when live-migration uses the scheduler to select a host, it constructs
a request_spec that has no extra_specs in it.

By making use of the existing helper method build_request_spec(), the
request to the scheduler will include extra_specs.

Change-Id: I5bc6c6418653c256a42da7b0a343086ec9863da1
Closes-Bug: #1224014