Merge remote-tracking branch 'remotes/xanclic/tags/pull-block-2018-08-31-v2' into staging

Block patches:
- (Block) job exit refactoring, part 1
  (removing job_defer_to_main_loop())
- test-bdrv-drain leak fix

# gpg: Signature made Fri 31 Aug 2018 15:30:33 BST
# gpg:                using RSA key F407DB0061D5CF40
# gpg: Good signature from "Max Reitz <mreitz@redhat.com>"
# Primary key fingerprint: 91BE B60A 30DB 3E88 57D1  1829 F407 DB00 61D5 CF40

* remotes/xanclic/tags/pull-block-2018-08-31-v2:
  jobs: remove job_defer_to_main_loop
  jobs: remove ret argument to job_completed; privatize it
  block/backup: make function variables consistently named
  jobs: utilize job_exit shim
  block/mirror: utilize job_exit shim
  block/commit: utilize job_exit shim
  jobs: add exit shim
  jobs: canonize Error object
  jobs: change start callback to run callback
  tests: fix bdrv-drain leak

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