* gnu/system/image.scm (mbr-disk-image, mbr-raw-image-type): New variables.
(qcow2-image-type): Inherit mbr-disk-image.
* guix/scripts/system.scm (%default-options): Use mbr-raw-image-type by
default.
* gnu/tests/install.scm (run-install): Use mbr-raw in the tests.
* doc/guix-cookbook.texi (Guix System Image API): Update the list of image
types.
* doc/guix.texi (Invoking guix system, System Images, image-type Reference):
Add mbr-raw and switch documented default to it.