Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2018-06-20-v2' into staging

nbd patches for 2018-06-20

Add experimental x-nbd-server-add-bitmap to expose a disabled
bitmap over NBD, in preparation for a pull model incremental
backup scheme. Also fix a corner case protocol issue with
NBD_CMD_BLOCK_STATUS, and add new NBD_CMD_CACHE.

- Eric Blake: tests: Simplify .gitignore
- Eric Blake: nbd/server: Reject 0-length block status request
- Vladimir Sementsov-Ogievskiy: 0/6 NBD export bitmaps
- Vladimir Sementsov-Ogievskiy: nbd/server: introduce NBD_CMD_CACHE

# gpg: Signature made Thu 21 Jun 2018 15:53:55 BST
# gpg:                using RSA key A7A16B4A2527436A
# 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-2018-06-20-v2:
  nbd/server: introduce NBD_CMD_CACHE
  docs/interop: add nbd.txt
  qapi: new qmp command nbd-server-add-bitmap
  nbd/server: implement dirty bitmap export
  nbd/server: add nbd_meta_empty_or_pattern helper
  nbd/server: refactor NBDExportMetaContexts
  nbd/server: fix trace
  nbd/server: Reject 0-length block status request
  tests: Simplify .gitignore

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