bhyve: move virtio-net header processing to pci_virtio_net

This patch cleans up the API between the net frontends (e1000,
virtio-net) and the net backends (tap and netmap).
We move the virtio-net header stripping/prepending to the
virtio-net code, where this functionality belongs.
In this way, the netbe_send() and netbe_recv() signatures
can have const struct iov * rather than struct iov *.

Reviewed by: grehan, bcr, aleksandr.fedorov@itglobal.com
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D23342