Allow vtnet operation without merged rx buffers.

[grehan]

Summary

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.

Test Plan

Tested by verifying that NetBSD 5.2.2, 6.1.4 and 7-beta
no longer generate an assert() in bhyve, and can access
the network via a virtio interface.

Verify that existing FreeBSD/Linux guests continue to work.

[tychon]

This looks good!

At some point to be actually be compliant with the merge-rx feature, a series of vq_getchain()/vq_relchain() operations culminating with a vrh_bufs > 1 header will need to happen.

[neel]

Looks good.

[grehan]

Agreed on the merge-rx stuff. Not sure the API can handle it the way it is written, but that can be changed.

[markj]

Committed in r271338.