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

Pull request

# gpg: Signature made Thu 30 Jan 2020 21:31:02 GMT
# gpg:                using RSA key 8695A8BFD3F97CDAAC35775A9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <[email protected]>" [full]
# gpg:                 aka "Stefan Hajnoczi <[email protected]>" [full]
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
  tests/qemu-iotests: use AIOMODE with various tests
  tests/qemu-iotests: enable testing with aio options
  qemu-nbd: adds option for aio engines
  qemu-img: adds option to use aio engine for benchmarking
  qemu-io: adds option to use aio engine
  block/io_uring: adds userspace completion polling
  block: add trace events for io_uring
  block/file-posix.c: extend to use io_uring
  blockdev: adds bdrv_parse_aio to use io_uring
  util/async: add aio interfaces for io_uring
  stubs: add stubs for io_uring interface
  block/io_uring: implements interfaces for io_uring
  block/block: add BDRV flag for io_uring
  qapi/block-core: add option for io_uring
  configure: permit use of io_uring
  block/io: take bs->reqs_lock in bdrv_mark_request_serialising
  block/io: wait for serialising requests when a request becomes serialising
  block: eliminate BDRV_REQ_NO_SERIALISING

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