Currently, when booting loopmounted iso file, it is necessary to
specify not only img_dev and img_loop (which should be sufficient),
but also archisolabel or archisodevice. With this patch, archisodevice
is directly populated with the correct loop device, and it is not
necessary to specify the label when booting from loopmounted iso,
which makes for leaner and cleaner grub.cfg.
