Fix tests/compute direct use of instance module objects

This switches the nova/tests/compute code to use nova.objects.Instance* vs
nova.objects.instance.Instance* to match the nova/compute changes in
the previous patch.

Partial-Blueprint: object-subclassing

Change-Id: I71f8705a22cbcb4ec171d80c1199bf28ccef3796
Merge "Fix tests/compute direct use of instance module objects"