bhyve: Store the FreeBSD OUI in little-endian in the controller data

Section 7.10.3 of the NVME 1.4b specification states that the IEEE OUI
in the identify controller structure is stored in little-endian format
(unlike the embedded OUI in EUI64 identifiers).

Reviewed by: corvink, chuck, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D41487