Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-06-13' into staging

nbd patches for 2019-06-13

- add 'qemu-nbd --pid-file'
- NBD-related iotest improvements
- NBD code refactoring in preparation for reconnect

# gpg: Signature made Thu 13 Jun 2019 16:37:58 BST
# gpg:                using RSA key A7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2019-06-13:
  block/nbd: merge NBDClientSession struct back to BDRVNBDState
  block/nbd: merge nbd-client.* to nbd.c
  block/nbd-client: drop stale logout
  nbd/server: Nicer spelling of max BLOCK_STATUS reply length
  iotests: Let 233 run concurrently
  iotests: Use qemu-nbd's --pid-file
  qemu-nbd: Do not close stderr
  iotests.py: Add qemu_nbd_early_pipe()
  qemu-nbd: Add --pid-file option

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