kvm: add ioeventfd and irqfd unregister support
kvm: add ioeventfd and irqfd unregister support

This is needed to support the plugin API, which might unregister such
eventfds.

BUG=chromium:800626
TEST=./build_test

Change-Id: Ic4e74ffd4b2374868644d1f6598509aba741ce00
Reviewed-on: https://chromium-review.googlesource.com/857908
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm