Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2016-09-20' into staging

Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2016-09-20' into staging

Block patches for 2.8

# gpg: Signature made Tue 20 Sep 2016 21:29:53 BST
# gpg:                using RSA key 0xF407DB0061D5CF40
# gpg: Good signature from "Max Reitz <mreitz@redhat.com>"
# Primary key fingerprint: 91BE B60A 30DB 3E88 57D1  1829 F407 DB00 61D5 CF40

* remotes/maxreitz/tags/pull-block-2016-09-20:
  iotest 055: refactor and speed up
  commit: get the overlay node before manipulating the backing chain
  blockdev: Modularize nfs block driver
  blockdev: Add dynamic module loading for block drivers
  blockdev: Add dynamic generation of module_block.h
  blockdev: prepare iSCSI block driver for dynamic loading
  qemu-img: add skip option to dd
  qemu-img: add the 'dd' subcommand

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