Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2021-01-20' into staging

Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2021-01-20' into staging

nbd patches for 2021-01-20

- minor resource leak fixes in qemu-nbd
- ensure proper aio context when nbd server uses iothreads
- iotest refactorings in preparation for rewriting ./check to be more
flexible, and preparing for more nbd server reconnect features

# gpg: Signature made Thu 21 Jan 2021 02:28:19 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-2021-01-20:
  iotests.py: qemu_io(): reuse qemu_tool_pipe_and_status()
  iotests.py: fix qemu_tool_pipe_and_status()
  iotests/264: fix style
  iotests: define group in each iotest
  iotests/294: add shebang line
  iotests: make tests executable
  iotests: fix some whitespaces in test output files
  iotests/303: use dot slash for qcow2.py running
  iotests/277: use dot slash for nbd-fault-injector.py running
  nbd/server: Quiesce coroutines on context switch
  block: Honor blk_set_aio_context() context requirements
  qemu-nbd: Fix a memleak in nbd_client_thread()
  qemu-nbd: Fix a memleak in qemu_nbd_client_list()

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