Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-09-02' into staging

Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-09-02' into staging

nbd patches for 2020-09-02

- fix a few iotests affected by earlier nbd changes
- avoid blocking qemu by nbd client in connect()
- build qemu-nbd for mingw

# gpg: Signature made Wed 02 Sep 2020 22:52:31 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-2020-09-02:
  nbd: disable signals and forking on Windows builds
  nbd: skip SIGTERM handler if NBD device support is not built
  block: add missing socket_init() calls to tools
  block/nbd: use non-blocking connect: fix vm hang on connect()
  iotests/259: Fix reference output
  iotests/059: Fix reference output

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