I suspect a KVM bug in Linux 4.17 though, because running the same
‘qemu-system-i386’ binary under strace works…  Perhaps worth applying
the patch above in the meantime, WDYT?

Anyway, with this patch I successfully run:

  guix system vm-image gnu/system/examples/bare-bones.tmpl -s i686-linux

and confirmed that the resulting image would boot through GRUB.

Ludo’.