Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2017-03-27' into staging

Block patches for 2.9-rc2.

# gpg: Signature made Mon 27 Mar 2017 16:47:54 BST
# gpg:                using RSA key 0xF407DB0061D5CF40
# gpg: Good signature from "Max Reitz <mreitz@redhat.com>"
# Primary key fingerprint: 91BE B60A 30DB 3E88 57D1  1829 F407 DB00 61D5 CF40

* remotes/maxreitz/tags/pull-block-2017-03-27:
  block/file-posix.c: Fix unused variable warning on OpenBSD
  file-posix: Make bdrv_flush() failure permanent without O_DIRECT
  nbd-client: fix handling of hungup connections
  qemu-img: print short help on getopt failure
  qemu-img: fix switch indentation in img_amend()
  qemu-img: show help for invalid global options

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