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

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

# gpg: Signature made Fri Mar 27 10:13:35 2015 GMT using RSA key ID 81AB73C8
# gpg: Good signature from "Stefan Hajnoczi <[email protected]>"
# gpg:                 aka "Stefan Hajnoczi <[email protected]>"

* remotes/stefanha/tags/block-pull-request:
  block: Document blockdev-add's immaturity
  qemu-iotests: Test unaligned 4k zero write
  block: Fix unaligned zero write
  nvme: Fix unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)

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