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

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

Block pull request

# gpg: Signature made Fri 25 Apr 2014 17:05:13 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:
  iscsi: Don't use error_is_set() to suppress additional errors
  blockdev: Clean up fragile use of error_is_set()
  nbd: Use return values instead of error_is_set(errp)
  qemu-img: Consistently name Error * objects err, and not errp
  Use error_is_set() only when necessary (again)
  block: Expose host_* drivers in blockdev-add
  MAINTAINERS: Add qemu-img/io to block subsystem
  qemu-iotests: Improve and make use of QMPTestCase.wait_until_completed()
  doc: add -drive rerror=,werror= to qemu --help output
  block: Prevent coroutine stack overflow when recursing in bdrv_open_backing_file.

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