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

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

# gpg: Signature made Wed 22 Mar 2017 17:28:56 GMT
# gpg:                using RSA key 0xBDBE7B27C0DE3057
# gpg: Good signature from "Jeffrey Cody <jcody@redhat.com>"
# gpg:                 aka "Jeffrey Cody <jeff@codyprime.org>"
# gpg:                 aka "Jeffrey Cody <codyprime@gmail.com>"
# Primary key fingerprint: 9957 4B4D 3474 90E7 9D98  D624 BDBE 7B27 C0DE 3057

* remotes/cody/tags/block-pull-request:
  blockjob: add devops to blockjob backends
  block-backend: add drained_begin / drained_end ops
  blockjob: add block_job_start_shim
  blockjob: avoid recursive AioContext locking

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