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

Pull request

# gpg: Signature made Fri 08 Mar 2019 16:53:34 GMT
# gpg:                using RSA key 9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>" [full]
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>" [full]
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
  iothread: document about why we need explicit aio_poll()
  iothread: push gcontext earlier in the thread_fn
  iothread: create main loop unconditionally
  iothread: create the gcontext unconditionally
  iothread: replace init_done_cond with a semaphore
  hw/block/virtio-blk: Clean req->dev repetitions
  MAINTAINERS: add missing support status fields

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