Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2018-11-19' into staging

nbd patches for 2018-11-19

Add iotest coverage for NBD connections using TLS, including
a couple of code fixes that it pointed out

- Mao Zhongyi: 0/3 Do some cleaning work in qemu-iotests
- Daniel P. Berrangé: io: return 0 for EOF in TLS session read after shutdown
- Daniel P. Berrangé: 0/6 Misc fixes to NBD
- Eric Blake: iotests: Drop use of bash keyword 'function'

# gpg: Signature made Mon 19 Nov 2018 17:43:32 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-2018-11-19:
  iotests: Drop use of bash keyword 'function'
  iotests: Also test I/O over NBD TLS
  tests: exercise NBD server in TLS mode
  tests: add iotests helpers for dealing with TLS certificates
  tests: check if qemu-nbd is still alive before waiting
  tests: pull qemu-nbd iotest helpers into common.nbd file
  io: return 0 for EOF in TLS session read after shutdown
  nbd/server: Ignore write errors when replying to NBD_OPT_ABORT
  nbd: fix whitespace in server error message
  qemu-iotests: Modern shell scripting (use $() instead of ``)
  qemu-iotests: convert `pwd` and $(pwd) to $PWD
  qemu-iotests: remove unused variable 'here'

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