Fix some Instance object class usage errors

This fixes a few places where Instance methods call
directly to Instance._from_db_object() instead of
using 'self' or 'cls' so that they can properly be
subclassed.

Related to blueprint compute-api-objects

Change-Id: I8e47901bd387048211d186f180991e8f780883c4
Merge "Fix some Instance object class usage errors"