adding default bdms is considering instances as volume backed

[Kaushik Chandrashekar]

There was a recent change in nova-api to add bdms for all instances. The device_name would be NULL by default
Instances with root_device_name as NULL are all considered to be volume backed instances because of the check

instance[root_device_name] == bdm[device_name]

We have to handle the scenario where they are NULL and return False.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/35465

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/35465
Committed: http://github.com/openstack/nova/commit/2da5dbf8423fb66357260219413c039250bc268f
Submitter: Jenkins
Branch: master

commit 2da5dbf8423fb66357260219413c039250bc268f
Author: Kaushik Chandrashekar <email address hidden>
Date: Wed Jul 3 03:55:12 2013 -0500

    Fix the is_volume_backed_instance check

    Changing the check for volume backed instance to handle instances with
    no root_device_name

    Fixes LP bug #1197237

    Change-Id: If0e2dff98a55ed1addf8a010849389f85db427f6