Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-03-08' into staging

Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-03-08' into staging

nbd patches for 2019-03-08

- support TLS client authorization in NBD servers
- iotest 223 race fix

# gpg: Signature made Fri 08 Mar 2019 17:37:59 GMT
# gpg:                using RSA key A7A16B4A2527436A
# 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-2019-03-08:
  iotests: Wait for qemu to end in 223
  nbd: fix outdated qapi docs syntax for tls-creds
  nbd: allow authorization with nbd-server-start QMP command
  qemu-nbd: add support for authorization of TLS clients

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