msmouse not recognized in guest

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1888663

The msmouse option for emulating a serial mouse does not seem to work in a
DOS guest.

I'm on Windows 10 X64, I have tried launching qemu (commit
d0cc248164961a7ba9d43806feffd76f9f6d7f41 but also way older) with:
./qemu-system-i386 -serial msmouse -fda mousetest.img
./qemu-system-i386 -chardev msmouse,id=msmouse -device isa-
serial,chardev=msmouse -fda mousetest.img
./qemu-system-i386 -chardev msmouse,id=msmouse -device pci-
serial,chardev=msmouse -chardev msmouse,id=msmouse -fda mousetest.img

Then I boot FreeDOS (but regular DOS shows same behavior), start the
CuteMouse driver and force the scan of a serial mouse with CTM /S.
The mouse is never found. With other drivers (in the attachment), the
mouse is probably not found but the driver is installed anyway, but it
does not work (there's a MOUSETST in the same floppy; it works iwth CTM
and PS/2 mouse emulation).

Using a serial port sniffer inside the guest, it would seem that data is
indeed transmitted. Setting a few printf in msmouse.c also confirms that
the mouse gets initilized and starts transmitting data. However, it does
not work...