Don't write a 1 to the RIRR bit in the IOAPIC redirection register. This bit

Don't write a 1 to the RIRR bit in the IOAPIC redirection register. This bit
is R/O, and although it should not matter what value is written there,
Hyper-V's emulated IOAPIC interprets a write of 1 in some unexpected way and
subsequently blocks interrupt delivery. This primarily manifests itself as
de(4) timeouts when using Hyper-V VMs with the "Legacy Network Adapter"
interface.

This diff has been in snaps for almost a month with no reported fallout.

Based on an idea originally from mikeb with further input from kettenis and
deraadt.