As requested in https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221074#c16, here's a stub hyper-v mouse driver that does nothing except for spamming the console on mouse events.
I have added the temporary module Makefile for this to make it easier to test.
What I am missing now is how to "make friends" with hms (guessing here) and feed the input report data into it, any hints? Should this driver somehow register on hidbus? Or simply add the hms as a child?