Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-02-25-v2' into staging

nbd patches for 2019-02-25

- iotest failure fixes for tests related to NBD

# gpg: Signature made Tue 26 Feb 2019 16:46:15 GMT
# 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-2019-02-25-v2:
  iotests: avoid broken pipe with certtool
  iotests: ensure we print nbd server log on error
  iotests: handle TypeError for Python 3 in test 242

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