libvirt: Fix kwargs for _create_image

Rescue uses the `suffix` and `disk_images` kwargs of `_create_image`, but
passes them in as positional args. We should use explicit kwargs here to be
clear that rescue depends on both of those parameters being present.

In addition, disk mapping was being passed in as kwarg, but is actually a
positional arg, so we should pass it in as a positional arg to be clear that
it's required.

Change-Id: Iaa990729b4bb3569f4c71ceaf1adabed58f73026
Merge "libvirt: Fix kwargs for _create_image"