MFC virtio-net changes.

Re-tested with NetBSD/amd64 5.2.2, 6.1.4 and 7-beta.

r271299:
Add a callback to be notified about negotiated features.

r271338:
Allow vtnet operation without merged rx buffers.

NetBSD's virtio-net implementation doesn't negotiate
the merged rx-buffers feature. To support this, check
to see if the feature was negotiated, and then adjust
the operation of the receive path accordingly by using
a larger iovec, and a smaller rx header.
In addition, ignore writes to the (read-only) status byte.

Approved by: re (glebius)
Obtained from: Vincenzo Maffione, Universita` di Pisa (r271299)