Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2018-01-17' into staging

pull-nbd-2018-01-17

- Vladimir Sementsov-Ogievskiy/Eric Blake: 0/6 NBD server refactoring

# gpg: Signature made Thu 18 Jan 2018 02:21:55 GMT
# 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-2018-01-17:
  nbd/server: structurize option reply sending
  nbd/server: Add helper functions for parsing option payload
  nbd/server: Add va_list form of nbd_negotiate_send_rep_err()
  nbd/server: Better error for NBD_OPT_EXPORT_NAME failure
  nbd/server: refactor negotiation functions parameters
  nbd/server: Hoist nbd_reject_length() earlier

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