[bug#28288] [PATCH 2/2] WIP

[Danny Milosavljevic]

> +    ;; XXX: 'guix-register' wants to palpate the things it registers, so
> +    ;; bind-mount the store on the target.
> +    (mkdir-p target-store)
> +    (mount (%store-directory) target-store "" MS_BIND)

What does this do?

gnu/build/vm.scm make-iso9660-image already provides /gnu/store a few lines 
down:

    (unless (zero? (apply system*
                          `(,grub-mkrescue "-o" ,target
                            ,(string-append "boot/grub/grub.cfg=" config-file)
                            ,(string-append "gnu/store=" os-drv "/..")
                                            ^^^^

Maybe I'm missing something...

Otherwise LGTM!