This makes hacking.py complain if someone adds something like:
from nova import db
into any of the files in nova/virt/* (aside from the fake.py file).
It also removes the rest of the dangling db imports that are no
longer needed.
Yay!
Change-Id: Iba3d53b87e65e33a55f8e5033b5d1d33b28d12f7