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.

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

Reviewed by:	neel, tychon
Phabric:	D745
MFC after:	3 days