docs/multiple-iothreads.txt: add documentation on IOThread programming

This document explains how IOThreads and the main loop are related,
especially how to write code that can run in an IOThread.  Currently
only virtio-blk-data-plane uses these techniques.  The next obvious
target is virtio-scsi; there has also been work on virtio-net.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
