vm_state and task_state not updated during instance delete

This bug was recently introduced during conversion of the
terminate_instance to the new object models.
https://review.openstack.org/#/c/36363
Fixed by calling the save method of the new model.
Also updated tests to use primitive instance so it is
converted to new object model by object_compat decorator
in the compute manager.

Related to: blueprint compute-api-objects
Fixes: bug #1212496
Change-Id: I4f4e45160008d3eb57f4f0935f13e22b6a561430