bhyve: implement NVMe SMART data I/O statistics

SMART data in NVMe includes statistics for number of read and write
commands issued as well as the number of "data units" read and written.
NVMe defines "data unit" as thousands of 512 byte blocks (e.g. 1 data
unit is 1-1,000 512 byte blocks, 3 data units are 2,001-3,000 512 byte
blocks).

This patch implements counters for:

• Data Units Read
• Data Units Written
• Host Read Commands
• Host Write Commands

and exposes the values when the guest reads the SMART/Health Log Page.

Fixes UNH Test 1.3.8

Tested by: Jason Tubnor
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24894