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

Pull request

These fixes make dataplane work again after the notify_me optimization was
added.  They also solve QEMUBH memory leaks and fix a bug in dataplane's
cleanup code.

# gpg: Signature made Wed Jul 29 14:50:26 2015 BST using RSA key ID 81AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>"

* remotes/stefanha/tags/block-pull-request:
  AioContext: force event loop iteration using BH
  AioContext: avoid leaking BHs on cleanup
  virtio-blk-dataplane: delete bottom half before the AioContext is freed

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