vm/image.bst: generate xz compressed raw image instead of qcow2

The resulting image is smaller to download (741M as I tested, fixes #203 (closed) I guess), can be reused for the iso images (See !520 (merged)), and xz can recreate sparse files when unpacking (The unpacked image is ~17G but only uses 3.5G on disk).

/cc @valentindavid