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

# gpg: Signature made Mon 16 Jan 2017 13:38:52 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:
  async: optimize aio_bh_poll
  aio: document locking
  aio-win32: remove walking_handlers, protecting AioHandler list with list_lock
  aio-posix: remove walking_handlers, protecting AioHandler list with list_lock
  aio: tweak walking in dispatch phase
  aio-posix: split aio_dispatch_handlers out of aio_dispatch
  qemu-thread: optimize QemuLockCnt with futexes on Linux
  aio: make ctx->list_lock a QemuLockCnt, subsuming ctx->walking_bh
  qemu-thread: introduce QemuLockCnt
  aio: rename bh_lock to list_lock
  block: get rid of bdrv_io_unplugged_begin/end

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