"dirty" callback in vt drivers

Hi,

I have implemented a simple hyper-v framebuffer driver (based on linux
hyperv_fb one) acting as a glue between hypervisor and vt_fb:

https://reviews.freebsd.org/D39395

All seems to be good, except that I am missing any sane way to know of
the "dirty" updates that should be sent to hypervisor, there do not seem
to be any callbacks in vt_fb.

At the moment, as a PoC, there's a loop at the end of the attach
callback that sends the "dirty the entire buffer" message to hypervisor
once per 1/20 second, and that works and does not seem to put any real
load on the host/guest, but still feels wrong.

Is there a better way for this glue driver to be notified of the updates?