Convert the if_vr(4) driver model to the interrupt filter model and use

Convert the if_vr(4) driver model to the interrupt filter model and use
a taskqueue.

This gives a 16% performance improvement under high load on slow systems,
especially when vr shares an interrupt with another device, which is
common with the Alix x86 boards.
Contrary to the other devices, I left the interrupt processing for loop
in because there was no significant difference in performance and this
should avoid enqueuing more taskqueues unnecessarily.
We also decided to move the vr_start_locked() call inside the for loop
because we found out that it helps performance since TCP ACKs now have a
chance to go out quicker.

Reviewed by:	yongari (older version, same idea)
Discussed with:	yongari, jhb