Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-10-27-v2' into staging

nbd patches for 2020-10-27

- Tweak the new block-export-add QMP command
- Allow multiple -B options for qemu-nbd
- Add qemu:allocation-depth metadata context as qemu-nbd -A
- Improve iotest use of NBD

# gpg: Signature made Fri 30 Oct 2020 20:22:42 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-2020-10-27-v2:
  nbd: Add 'qemu-nbd -A' to expose allocation depth
  nbd: Add new qemu:allocation-depth metadata context
  block: Return depth level during bdrv_is_allocated_above
  nbd: Allow export of multiple bitmaps for one device
  nbd: Refactor counting of metadata contexts
  nbd: Simplify qemu bitmap context name
  nbd: Update qapi to support exporting multiple bitmaps
  nbd: Utilize QAPI_CLONE for type conversion
  qapi: Add QAPI_LIST_PREPEND() macro
  block: Simplify QAPI_LIST_ADD
  iotests/291: Stop NBD server
  iotests/291: Filter irrelevant parts of img-info

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