Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2021-02-12' into staging

nbd patches for 2021-02-12

- let qemu-nbd handle larger backlog of connecting clients
- fix a few NBD-related iotest failures
- add block cancellation hook for faster response to NBD failures

# gpg: Signature made Fri 12 Feb 2021 19:57:56 GMT
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2021-02-12:
  iotests/264: add backup-cancel test-case
  block/backup: implement .cancel job handler
  iotests/264: add mirror-cancel test-case
  iotests.py: qemu_nbd_popen: remove pid file after use
  iotests/264: move to python unittest
  block/mirror: implement .cancel job handler
  job: add .cancel handler for the driver
  block/raw-format: implement .bdrv_cancel_in_flight handler
  block/nbd: implement .bdrv_cancel_in_flight
  block: add new BlockDriver handler: bdrv_cancel_in_flight
  io: error_prepend() in qio_channel_readv_full_all() causes segfault
  iotests/210: Fix reference output
  qemu-nbd: Permit --shared=0 for unlimited clients
  qemu-nbd: Use SOMAXCONN for socket listen() backlog

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