Fix nova/compute direct use of instance module objects

This switches the nova/compute code to use nova.objects.Instance* vs
nova.objects.instance.Instance* so that subclassing of objects will work
properly.

Partial-Blueprint: object-subclassing

Change-Id: I941ebb14caeb29fe55bb93bccaf95fd158024d34
Merge "Fix nova/compute direct use of instance module objects"