Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging

# gpg: Signature made Thu Apr  9 10:55:11 2015 BST using RSA key ID 81AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>"

* remotes/stefanha/tags/block-pull-request:
  block/iscsi: handle zero events from iscsi_which_events
  aio: strengthen memory barriers for bottom half scheduling
  virtio-blk: correctly dirty guest memory
  qcow2: Fix header update with overridden backing file

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