bhyve: set EV_CLEAR for EVFILT_VNODE mevents

When an EVFILT_VNODE filter event is triggered, reset it.

This fixes the issue where a virtio-blk resize event would cause the
mevent thread to consume 100% of the cpu.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D33326

(cherry picked from commit 0b29683b32df0233a85f597836e61cc6432a3e4f)