Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging

# gpg: Signature made Wed 04 Jan 2017 13:29:09 GMT
# gpg:                using RSA key 0x9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
  iothread: add poll-grow and poll-shrink parameters
  aio: self-tune polling time
  virtio: disable virtqueue notifications during polling
  aio: add .io_poll_begin/end() callbacks
  virtio: turn vq->notification into a nested counter
  virtio-scsi: suppress virtqueue kick during processing
  virtio-blk: suppress virtqueue kick during processing
  iothread: add polling parameters
  linux-aio: poll ring for completions
  virtio: poll virtqueues for new buffers
  aio: add polling mode to AioContext
  aio: add AioPollFn and io_poll() interface
  aio: add flag to skip fds to aio_dispatch()
  HACKING: document #include order

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>