Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-07-28' into staging

nbd patches for 2020-07-28

- fix NBD handling of trim/zero requests larger than 2G
- allow no-op resizes on NBD (in turn fixing qemu-img convert -c into NBD)
- several deadlock fixes when using NBD reconnect

# gpg: Signature made Tue 28 Jul 2020 15:59:42 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-2020-07-28:
  block/nbd: nbd_co_reconnect_loop(): don't sleep if drained
  block/nbd: on shutdown terminate connection attempt
  block/nbd: allow drain during reconnect attempt
  block/nbd: split nbd_establish_connection out of nbd_client_connect
  iotests: Test convert to qcow2 compressed to NBD
  iotests: Add more qemu_img helpers
  iotests: Make qemu_nbd_popen() a contextmanager
  block: nbd: Fix convert qcow2 compressed to nbd
  nbd: Fix large trim/zero requests

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