There are some virtio features that could improve performance; we should look at adding these if we think they'll help.

In no particular order:
1) VIRTIO_NET_F_MRG_RXBUF
Allows the guest to provide smaller buffers to the host, instead of the 64k monster buffers that are required when UFO/TSO features are turned on.
2) VIRTIO_RING_F_EVENT_IDX
Helps avoid triggering interrupts when the guest should be polling for received packets.
3) VIRTIO_RING_F_INDIRECT_DESC
Indirect descriptors support.