system: image: Add image-type support.

system: image: Add image-type support.

* gnu/system/image.scm (image-with-os): New macro. Rename the old
"image-with-os" procedure to ...
(image-with-os*): ... this new procedure,
(system-image): adapt according,
(raw-image-type, iso-image-type, uncompressed-iso-image-type
%image-types): new variables,
(lookup-image-type-by-name): new procedure.
(find-image): remove it.
* gnu/system/images/hurd.scm (hurd-image-type): New variable,
use it to define ...
(hurd-disk-image): ... this variable, using "os->image" procedure.
* gnu/tests/install.scm (run-install): Rename
installation-disk-image-file-system-type parameter to installation-image-type,
use os->config instead of find-image to compute the image passed to system-image,
(%test-iso-image-installer) adapt accordingly,
(guided-installation-test): ditto.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>