bhyve: add basic NVMe Firmware Commit support

This commit updates the Identify Controller data to advertise the
Controller supports a single firmware slot and that firmware slot 1 is
read-only. Additionally, it returns an "Invalid Firmware Slot" error
when the host issues any Firmware Commit command (a.k.a. Firmware
Activate).

Fixes UNH Test 5.5.3

Tested by: Jason Tubnor
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24897