Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-01-05' into staging

Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-01-05' into staging

nbd patches for 2019-01-05

Error and trace improvements in NBD code, such as less noise for
common disconnect scenarios.

- Vladimir Sementsov-Ogievskiy: 0/3 nbd-client: drop extra error noise
- Eric Blake: portions of 0/22 nbd: add qemu-nbd --list

# gpg: Signature made Sat 05 Jan 2019 13:58:54 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-2019-01-05:
  nbd/client: Drop pointless buf variable
  qemu-nbd: Fail earlier for -c/-d on non-linux
  nbd/client: More consistent error messages
  nbd: Document timeline of various features
  qemu-nbd: Use program name in error messages
  block/nbd-client: use traces instead of noisy error_report_err
  nbd/client: Trace all server option error messages
  nbd: publish _lookup functions

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