Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer patches:

- block: Fix forbidden use of polling in drained_end
- block: Don't wait for I/O throttling while exiting QEMU
- iotests: Use read-zeroes for the null driver to be Valgrind-friendly

# gpg: Signature made Fri 19 Jul 2019 14:30:14 BST
# gpg:                using RSA key 7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>" [full]
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
  iotests: Test quitting with job on throttled node
  vl: Drain before (block) job cancel when quitting
  iotests: Test commit with a filter on the chain
  iotests: Add @has_quit to vm.shutdown()
  block: Loop unsafely in bdrv*drained_end()
  tests: Extend commit by drained_end test
  block: Do not poll in bdrv_do_drained_end()
  tests: Lock AioContexts in test-block-iothread
  block: Make bdrv_parent_drained_[^_]*() static
  block: Add @drained_end_counter
  tests: Add job commit by drained_end test
  block: Introduce BdrvChild.parent_quiesce_counter
  iotests: Set read-zeroes on in null block driver for Valgrind

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