bhyve: virtio shares definitions between sys/dev/virtio

Definitions inside usr.sbin/bhyve/virtio.h are thrown away.
Definitions in sys/dev/virtio are used instead.

This reduces code duplication.

Sponsored by: The FreeBSD Foundation
Reviewed by: grehan
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D29084