Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-09-24-v2' into staging

nbd patches for 2019-09-24

- Improved error message for plaintext client of encrypted server
- Fix various assertions when -object iothread is in use
- Silence a Coverity error for use-after-free on error path

# gpg: Signature made Wed 25 Sep 2019 14:35:52 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-2019-09-24-v2:
  util/qemu-sockets: fix keep_alive handling in inet_connect_saddr
  tests: Use iothreads during iotest 223
  nbd: Grab aio context lock in more places
  nbd/server: attach client channel to the export's AioContext
  nbd/client: Add hint when TLS is missing

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