Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2017-07-17' into staging

nbd patches for 2017-07-17

- Eric Blake: nbd: Fix iotests failure due to changed client error message
- Eric Blake: [0/2] NBD fixes before softfreeze

# gpg: Signature made Mon 17 Jul 2017 23:12:20 BST
# gpg:                using RSA key 0xA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>"
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>"
# gpg:                 aka "[jpeg image of size 6874]"
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2017-07-17:
  nbd: Fix server reply to NBD_OPT_EXPORT_NAME of older clients
  nbd: Trace client command being sent
  nbd: Fix iotests failure due to changed client error message

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