Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2018-09-26' into staging

nbd patches for 2018-09-26

Fixes for external clients; add reminder to revisit naming of x- command

- Vladimir Sementsov-Ogievskiy: nbd/server: send more than one extent of base:allocation context
- John Snow: qapi: bitmap-merge: document name change
- Vladimir Sementsov-Ogievskiy: nbd/server: fix bitmap export

# gpg: Signature made Thu 27 Sep 2018 03:40:03 BST
# gpg:                using RSA key A7A16B4A2527436A
# gpg: Good signature from "Eric Blake <[email protected]>"
# gpg:                 aka "Eric Blake (Free Software Programmer) <[email protected]>"
# 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-2018-09-26:
  nbd/server: send more than one extent of base:allocation context
  qapi: bitmap-merge: document name change
  nbd/server: fix bitmap export

Signed-off-by: Peter Maydell <[email protected]>