- Folks in IRC logs had mentioned that Shepherd added support for the "service" interface in 0.10 (
https://logs.guix.gnu.org/guix/2024-03-12.log#133456). I took a stab that maybe there was a 0.9 compatibility layer that was removed at some point between the release of 0.10 and the current day, so I did a `guix time-machine --commit=72a0d835 -- pull` and reconfigure. That did appear to get around the "service" interface Shepherd error that OP mentioned in their report.
- For the magic number disk issue, it appears that the bootloader is mistakenly (???) configured to boot from /dev/vda1 with the grub-bootloader. When I changed it to /dev/vda2 (the root partition of the QEMU image) it booted successfully. /dev/vda1 appears to be an EFI partition, so maybe `grub-efi-bootloader` should be used instead?