Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2018-04-16' into staging

A fix for handling dirty bitmaps stored in qcow2 files.  This is not
absolutely necessary for 2.12, but if there is an rc4, it should go in.

# gpg: Signature made Mon 16 Apr 2018 13:35:08 BST
# gpg:                using RSA key F407DB0061D5CF40
# 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-2018-04-16:
  iotests: fix 169
  qcow2: try load bitmaps only once

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