bhyve: Add AER support to NVMe emulation

This adds support to bhyve's NVMe device emulation for processing Async
Event Requests but not returning them (i.e. Async Event Notifications).

Fixes UNH Test 5.5.2

Tested by: Jason Tubnor
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24896