Don't use instance['instance_type'] for scheduler filters in migration

Instead, pull a fresh copy from the database, which has the proper
extra_specs cleanup which everyone downstream of us expects.

Fixes bug 1136430

Change-Id: Ifcb9feff8291096eed7c198b8e7a35c3c38ca93e
Merge "Don't use instance['instance_type'] for scheduler filters in migration"