Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-07-13' into staging

NBD patches for 2020-07-13

- fix off-by-one truncation in corner-case name display
- use fcntl correctly
- iotest cleanups that enable testing an upcoming fix for NBD close

# gpg: Signature made Mon 13 Jul 2020 15:11:35 BST
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# 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-2020-07-13:
  iotests.py: filter_testfiles(): filter SOCK_DIR too
  iotests.py: QemuIoInteractive: print output on failure
  iotests: QemuIoInteractive: use qemu_io_args_no_fmt
  hax: Fix setting of FD_CLOEXEC
  nbd: Avoid off-by-one in long export name truncation

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