Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2021-07-09-v2' into staging

nbd patches for 2021-07-09

- enhance 'qemu-img map --output=json' to make it easier to duplicate
backing chain allocation patterns
- fix a race in the 'yank' QMP command in relation to NBD requests

# gpg: Signature made Mon 12 Jul 2021 18:43:37 BST
# 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-2021-07-09-v2:
  nbd: register yank function earlier
  qemu-img: Reword 'qemu-img map --output=json' docs
  qemu-img: Make unallocated part of backing chain obvious in map
  iotests: Improve and rename test 309 to nbd-qemu-allocation

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