qemu: conf: Improve the foo_image_format setting descriptions
The current description of the various foo_image_format settings can
be construded to imply the setting is only used to control compression
of the image. Improve the documentation to clarify that format describes
the representation of guest memory blocks on disk, which includes
compression among other possible layouts.
Signed-off-by: Jim Fehlig <jfehlig@suse.com> Reviewed-by: Martin Kletzander <mkletzan@redhat.com>