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

Block layer patches for 2.6

# gpg: Signature made Tue 05 Apr 2016 16:32:25 BST using RSA key ID C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"

* remotes/kevin/tags/for-upstream:
  crypto: Avoid memory leak on failure
  qemu-iotests: 149: Use "/usr/bin/env python"
  block: Forbid I/O throttling on nodes with multiple parents for 2.6
  block: forbid x-blockdev-del from acting on DriveInfo

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