Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-05-18' into staging

nbd patches for 2020-05-20

- fix stranded fd in 'qemu-nbd -c /dev/nbd0'
- add 'qemu-img map --start-offset --max-length' options

# gpg: Signature made Mon 18 May 2020 17:29:56 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-2020-05-18:
  iotests: Enhance 223 to cover qemu-img map improvements
  qemu-img: Add --start-offset and --max-length to map
  qemu-img: refactor dump_map_entry JSON format output
  qemu-img: validate image length in img_map
  qemu_img: add cvtnum_full to print error reports
  qemu-nbd: Close inherited stderr

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