Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2019-06-14' into staging

Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2019-06-14' into staging

Block patches:
- Allow blockdev-backup from nodes that are not in qemu's main AIO
  context to newly added nodes
- Add salvaging mode to qemu-img convert
- Minor fixes to tests, documentation, and for less Valgrind annoyance

# gpg: Signature made Fri 14 Jun 2019 14:38:11 BST
# gpg:                using RSA key 91BEB60A30DB3E8857D11829F407DB0061D5CF40
# gpg:                issuer "mreitz@redhat.com"
# gpg: Good signature from "Max Reitz <mreitz@redhat.com>" [full]
# Primary key fingerprint: 91BE B60A 30DB 3E88 57D1  1829 F407 DB00 61D5 CF40

* remotes/maxreitz/tags/pull-block-2019-06-14:
  iotests: Test qemu-img convert -C --salvage
  iotests: Test qemu-img convert --salvage
  blkdebug: Inject errors on .bdrv_co_block_status()
  blkdebug: Add "none" event
  blkdebug: Add @iotype error option
  qemu-img: Add salvaging mode to convert
  qemu-img: Move quiet into ImgConvertState
  blockdev: Overlays are not snapshots
  qapi/block-core: Overlays are not snapshots
  qemu-img: Fix options leakage in img_rebase()
  iotests: restrict 254 to support only qcow2
  hw/block/fdc: floppy command FIFO memory initialization
  iotests: Fix intermittent failure in 219
  iotests: Filter 175's allocation information
  event_match: always match on None value
  iotests: add iotest 256 for testing blockdev-backup across iothread contexts
  iotests.py: rewrite run_job to be pickier
  QEMUMachine: add events_wait method
  iotests.py: do not use infinite waits
  blockdev-backup: don't check aio_context too early

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