virtio: Support non-legacy network device and queue

The non-legacy interface always defines num_buffers in the header,
regardless of whether VIRTIO_NET_F_MRG_RXBUF, just leaving it unused. We
also need to ensure our virtqueue doesn't filter out VIRTIO_F_VERSION_1
during negotiation, as it supports non-legacy transports just fine. This
fixes network packet transmission on TinyEMU.

Reviewed by: br, brooks (mentor), jhb (mentor)
Approved by: br, brooks (mentor), jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D25132