Libvirt: Added suffix to configdrive_path required for rescue

It was observed that during nova rescue, nova failed to create
a rescue disk for disk.config. The reason being that .rescue
suffix was missing from the configdrive path. It was working
till Havana but the functionality broke in Icehouse. This commit
fixes the suffix problem.

Unittests have been added to verify following scenarios:
    1. Make sure that .rescue disks are created when not using
        config drive
    2. Make sure that .rescue disks are created when using
        config drive

Closes-Bug: #1334024

Change-Id: I87449ffddd047cb84b7b881757ea4c29927b95da
Merge "Libvirt: Added suffix to configdrive_path required for rescue"