bhyve: Fix assignment of *niov2 in split_iov()

niov2 returns the number of entries in the iovec starting at offset
"offset". Here we are unconditionally setting it to 1, which of course
isn't right.

Fixes: a28cf86c4171 ("bhyve/virtio: Rework iovec handling functions for efficiency and clarity")
Reported by: Claude and Ada Logics
Reviewed by: Hans Rosenfeld <rosenfeld@grumpf.hope-2000.org>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58625