Rescue not using base images

[Rick Harris]

Rescue mode should use the 'base' image of an instance (the image it was originally built off of) not the 'current' image, since that likely is broken (hence the use of rescue).

The code right now doing the wrong thing and using the current image. The solution is to fix this in both the hypervisor agnostic compute-manager code and also in some places in the XenServer driver where we hard-code it to use the current image instead of the image that was passed into the function.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/15521

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/15521
Committed: http://github.com/openstack/nova/commit/9f59d4af0dd06f8a70e2a647c5932472e0bf0f3a
Submitter: Jenkins
Branch: master

commit 9f59d4af0dd06f8a70e2a647c5932472e0bf0f3a
Author: Rick Harris <email address hidden>
Date: Tue Nov 6 19:09:06 2012 +0000

    Use base image for rescue instance.

    Rescue mode was using the instance's current image when building the
    rescue VM. This is a problem because that image may be broken, hence the
    need for the rescue operation.

    Rescue should use the 'base' image instead.

    Fixes bug 1075701

    Change-Id: Ie3e65cc75c4710f7b6391d5111cb096755e600d4