Expand the tests structure in test_small(..) to workaround the
"initializer not constant" warning with gcc

Approved by: jmmv (mentor)
MFC after: 3 days
Phabric: D744
Sponsored by: EMC / Isilon Storage Division
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
Bump __FreeBSD_version after SA-14:18

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation
Add diagnostic printfs and disable test # 4 on i386

Reviewed by: jmmv, rpaulo
MFC after: 3 days
Phabric: D749
PR: 191676
Sponsored by: EMC / Isilon Storage Division