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

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

# gpg: Signature made Tue 11 Apr 2017 13:10:55 BST
# gpg:                using RSA key 0xCA35624C6A9171C6
# gpg: Good signature from "Fam Zheng <[email protected]>"
# gpg: WARNING: This key is not certified with sufficiently trusted signatures!
# gpg:          It is not certain that the signature belongs to the owner.
# Primary key fingerprint: 5003 7CB7 9706 0F76 F021  AD56 CA35 624C 6A91 71C6

* remotes/famz/tags/block-pull-request:
  sheepdog: Use bdrv_coroutine_enter before BDRV_POLL_WHILE
  block: Fix bdrv_co_flush early return
  block: Use bdrv_coroutine_enter to start I/O coroutines
  qemu-io-cmds: Use bdrv_coroutine_enter
  blockjob: Use bdrv_coroutine_enter to start coroutine
  block: Introduce bdrv_coroutine_enter
  async: Introduce aio_co_enter
  coroutine: Extract qemu_aio_coroutine_enter
  tests/block-job-txn: Don't start block job before adding to txn
  block: Quiesce old aio context during bdrv_set_aio_context
  block: Make bdrv_parent_drained_begin/end public

Signed-off-by: Peter Maydell <[email protected]>