Add +c compression attr to root.img at install/creation time

[tasket]

Idea:
Disk space on Qubes can benefit from transparent root.img compression in template and standalone vms: The content type and usage pattern is a good match.

Implementation:
Linux supports selective flagging of files for compression, and filesystems like btrfs support it. This makes the feature easy to implement (set +c attribute and it will be either used or ignored depending on host fs) with one snag: The attribute must be set at creation time to achieve compression of the entire image file, and this implies that file append/concatenation must be used when creating the image (the latter already happens to be true).

There should also be a global setting for this with an appropriate default.