bhyve: fix byte order for manually set NVMe eui64

Manually specified eui64 value gets converted to big endian twice:
first using htobe64() and then using be64enc(). On little-endian hosts
that results in a little-endian value instead of a big-endian.

Fix by removing htobe64() for a user submitted value.

Fixes: 409a80e5a434 ("bhyve: Create EUI64 for NVMe namespaces")
Reviewed by: chuck
Relnotes: yes
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D59080