virtio-net: read directly from the backend to the descriptor chain

[slp]

Right now, we're using intermediate buffers when reading from the backends, and then writing to the descriptor chain. For backends capable of using readv, we should translate the descriptor chain into an iovec and use it to do the operation in a single copy.

[slp]

@mtjhrc please let me know if you want to tackle this one yourself.

[mtjhrc]

Yeah I can work on this later.

I'm thinking maybe we should have it as some generic utility/abstraction for dealing with IO <-> descriptor chain reading/writing, It should also be useful for other devices. Anyways starting with virtio-net makes sense.

[nirs]

See also #405