Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2018-03-13-v2' into staging

Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2018-03-13-v2' into staging

nbd patches for 2018-03-13

- Eric Blake: iotests: Fix stuck NBD process on 33
- Vladimir Sementsov-Ogievskiy: 0/5 nbd server fixing and refactoring before BLOCK_STATUS
- Eric Blake: nbd/server: Honor FUA request on NBD_CMD_TRIM
- Stefan Hajnoczi: 0/2 block: fix nbd-server-stop crash after blockdev-snapshot-sync
- Vladimir Sementsov-Ogievskiy: nbd block status base:allocation

# gpg: Signature made Tue 13 Mar 2018 20:48:37 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-2018-03-13-v2:
  iotests: new test 209 for NBD BLOCK_STATUS
  iotests: add file_path helper
  iotests.py: tiny refactor: move system imports up
  nbd: BLOCK_STATUS for standard get_block_status function: client part
  block/nbd-client: save first fatal error in nbd_iter_error
  nbd: BLOCK_STATUS for standard get_block_status function: server part
  nbd/server: add nbd_read_opt_name helper
  nbd/server: add nbd_opt_invalid helper
  iotests: add 208 nbd-server + blockdev-snapshot-sync test case
  block: let blk_add/remove_aio_context_notifier() tolerate BDS changes
  nbd/server: Honor FUA request on NBD_CMD_TRIM
  nbd/server: refactor nbd_trip: split out nbd_handle_request
  nbd/server: refactor nbd_trip: cmd_read and generic reply
  nbd/server: fix: check client->closing before sending reply
  nbd/server: fix sparse read
  nbd/server: move nbd_co_send_structured_error up
  iotests: Fix stuck NBD process on 33

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