Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2017-11-17' into staging

nbd patches for 2017-11-17

Eric Blake - nbd: Don't crash when server reports NBD_CMD_READ failure
Eric Blake - nbd/client: Use error_prepend() correctly
Eric Blake - nbd/client: Don't hard-disconnect on ESHUTDOWN from server
Eric Blake - nbd/server: Fix error reporting for bad requests

# gpg: Signature made Fri 17 Nov 2017 14:53:30 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-2017-11-17:
  nbd/server: Fix error reporting for bad requests
  nbd/client: Don't hard-disconnect on ESHUTDOWN from server
  nbd/client: Use error_prepend() correctly
  nbd: Don't crash when server reports NBD_CMD_READ failure

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