Fix driver tests on Windows
Fix driver tests on Windows

Driver tests were unable to run on Windows due to some import
errors.

Specifically, the 'setUp' method of the 'PoisonFunctions'
class would import 'nova.virt.libvirt' which in turn would cause
an error when running the Windows tests.

This change fixes this issue.

Partial-Bug: #1736395

Change-Id: Id21a24257ec1e00a1eb942aec725ae76e9f6e688

mahak/nova,klmitch/nova,klmitch/nova,klmitch/nova,mahak/nova,rahulunair/nova,rahulunair/nova,openstack/nova,klmitch/nova,openstack/nova,rahulunair/nova,mahak/nova,openstack/nova