Remove unnecessary cleanup in test

The test test_rollback_live_migration_at_destination_correctly
makes use of a fake instance object. No need to clean this from
the database with db.instance_destroy.

This issue was also raised in:
https://review.openstack.org/#/c/90801/3/nova/tests/compute/test_compute.py

Change-Id: I66a9f60b9e7b2f69f79976c03e4e9ddcc5e02993
Check object's field

A nova object should have all fields as instance of fields.Field.
This patch adds a check on the metaclass to make sure of it.

This check happens only when class import so should have no
runtime performance impact.

Change-Id: I2cdf7eb097fb6907cbd048fa73138b352ab547d7
Merge "Check object's field"
Merge "Remove unnecessary cleanup in test"