Block fixes for 1.7.0

# gpg: Signature made Fri 15 Nov 2013 09:51:25 AM PST using RSA key ID C88F2FD6
# gpg: Can't check signature: public key not found

# By Max Reitz (3) and others
# Via Kevin Wolf
* kwolf/tags/for-anthony:
  block: Fail if requested driver is not available
  MAINTAINERS: add block driver sub-maintainers
  qemu-img: Fix overwriting 'ret' before using
  qemu-iotests: Test qcow2 count_contiguous_clusters()
  qcow2: fix possible corruption when reading multiple clusters
  qmp: access the local QemuOptsLists for drive option
  MAINTAINERS: add block tree repo URLs
  qemu-iotests: Extend 041 for unbacked mirroring
  block/drive-mirror: Check for NULL backing_hd
  qapi-schema: Update description for NewImageMode
  block: Print its file name if backing file opening failed

Message-id: 1384537999-5972-1-git-send-email-kwolf@redhat.com
Signed-off-by: Anthony Liguori <aliguori@amazon.com>