virtio: Support non-legacy network device and queue

[jrtc27]

Summary

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.

Test Plan

Network interface still works in QEMU (which provides a legacy device by
default), but now also works on a TinyEMU-based system.