Add a new bhyve network backend that allow to connect the VM to the netgraph(4) network.
The backend uses the socket API with the PF_NETGRAPH protocol family, which is provided by the ng_socket(4).

To use the new backend, provide the following bhyve option:
-s X:Y:Z,[virtio-net|e1000],netgraph,socket=[ng_socket name],path=[destination node],hook=[our socket src hook],peerhook=[dst node hook]

Reviewed by: vmaffione, lutz_donnerhacke.de
Approved by: vmaffione (mentor)
Sponsored by: vstack.com
Differential Revision: https://reviews.freebsd.org/D24620