Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2018-02-09' into staging

nbd patches for 2018-02-09

- Vladimir Sementsov-Ogievskiy: iotests: 205: support luks format
- Eric Blake: block: Simplify bdrv_can_write_zeroes_with_unmap()

# gpg: Signature made Fri 09 Feb 2018 18:34:20 GMT
# 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-2018-02-09:
  block: Simplify bdrv_can_write_zeroes_with_unmap()
  iotests: 205: support luks format

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