bhyve: avoid TOCTOU on iov_len in virtio_vq_recordon()

Avoid a race condition when accessing guest memory, by reading memory
contents only once.

This has also been applied to _vq_record() in
sys/dev/beri/virtio/virtio.c, as per markj@'s suggestion.

Reported by: Synacktiv
Reviewed by: markj
Security: HYP-10
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45735