Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Mirror block job fixes for 2.6.0-rc3

# gpg: Signature made Wed 20 Apr 2016 15:56:43 BST using RSA key ID C88F2FD6
# gpg: Good signature from "Kevin Wolf <[email protected]>"

* remotes/kevin/tags/for-upstream:
  iotests: Test case for drive-mirror with unaligned image size
  iotests: Add iotests.image_size
  mirror: Don't extend the last sub-chunk
  block/mirror: Refresh stale bitmap iterator cache
  block/mirror: Revive dead yielding code

Signed-off-by: Peter Maydell <[email protected]>