libvirt: Get driver type from base image type.

If we are using the raw backend (no cow), we should set the driver
type based on the type of the source image instead of always
using raw.

Calls to to_xml were moved after _create_image so that the image
type could be determined when creating the xml for libvirt.

Fixes bug 1163009

Change-Id: Ic8d5f0ab83d868a42f834d39c0afb64818d7e027
Merge "libvirt: Get driver type from base image type."