Make _downsize_quota_delta() use stashed instance types

This change changes the _downsize_quota_delta() method to take instance
as a param instead of migration_ref, and use it to extract the old and new
instance_types instead of looking them up in the database. This is needed
for this case since the method can be called from compute manager during
the periodic task to confirm pending migrations, and thus access to the
database may not be allowed.

Fixes bug #1158897

Change-Id: I647c51bc4d30e6dfd62abe7b65a9109d6aa2bb6f