FreeDOS / MS-Dos / Windows 3.11 cannot perform reboot with 'isapc' machine

I was installing MS-Dos 6.22 + Windows 3.11 in preparation for running Microsoft Bob, and noticed that when they try to perform a reboot, they just get stuck. The console cursor stays flashing on/off, but the DOS prompt no longer responds to input.

It is fairly easy to reproduce, even FreeDOS is impacted - download the FreeDOS bootable CDROM image, then

$ qemu-img create demo.img 100MB

$ qemu-system-x86_64 -machine isapc -cdrom ~/Downloads/FD12CD.iso  -hda demo.img  -monitor stdio

Wait for the installer to startup, and then in the monitor console run

  sendkey ctrl-alt-delete

It will fail to reboot

Testing shows this is a regression from QEMU 2.8.0 onwards, and git bisect further narrowed it down to a SEABIOS update

commit 6e99f5741ff1b408ea76e6caf2bd4f76df4060e9 (HEAD, tag: pull-seabios-20161027-2, tag: pull-seabios-20161027-1, refs/bisect/bad)
Author: Gerd Hoffmann <email address hidden>
Date:   Thu Oct 27 16:42:28 2016 +0200

    seabios: update to 1.10.0 release.

Note that this seems particular to the "isapc" machine type - with the "pc" machine type, reboot still works