bhyve: add support for ng_device network backend

Signed-off-by: Quentin Thébault <quentin.thebault@defenso.fr>

Reviewed by: markj
MFC after: 1 month
Sponsored by: Defenso
Differential Revision: https://reviews.freebsd.org/D52542
Pull Request: https://github.com/freebsd/freebsd-src/pull/1880

(cherry picked from commit 1aad95345237424918e5f6b18464df4dbc2aa1d8)