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

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

# gpg: Signature made Mon 09 Nov 2015 10:08:17 GMT 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:
  blockdev: acquire AioContext in hmp_commit()
  monitor: add missed aio_context_acquire into vm_completion call
  aio: Introduce aio-epoll.c
  aio: Introduce aio_context_setup
  aio: Introduce aio_external_disabled
  dataplane: support non-contigious s/g
  dataplane: simplify indirect descriptor read

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