image: Do not use VM to create disk-images.

image: Do not use VM to create disk-images.

Now that installing Grub on raw disk-images is supported, we do not need to
rely on (gnu system vm) module.

* gnu/system/image.scm (make-system-image): Rename to ...
(system-image): ... this, and remove the compatibility wrapper.
(find-image): Turn to a monadic procedure. This will become useful when
introducing Hurd support, to be able to detect the target system.
* gnu/ci.scm (qemu-jobs): Use lower-object now that system-image returns a
file-like object.
* gnu/tests/install.scm (run-install): Ditto.
* guix/scripts/system.scm (system-derivation-for-action): Add a 'base-image'
argument,
(perform-action): adapt accordingly.