Also, the Linux kernel console is something independent from grub.

To test that, try adding "console=ttyS0,115200" to the operating-system's
kernel-arguments.

Currently, Guix automatically set up a getty if it detects the
"console=ttyS0" argument in /proc/cmdline .  That is supposed
to ensure some reasonable console continuity, so as soon as the Linux
kernel has a console, the userspace is also supposed to use it automatically.

It might make sense to make our grub automatically add "console=ttyS0,115200"
to the Linux command line that it already emits.