Migrate NovaPersistentObject and ObjectListBase to Fields
Migrate NovaPersistentObject and ObjectListBase to Fields

This actually uncovers an invalid testing scenario we had for lists,
which was using non-objects in the list-of-objects tests.

Related to blueprint structured-object-fields

Change-Id: I94609c1fd4a8b011e71bde7d9510ec2d3cb29868

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects