Block layer fixes for 2.9.0-rc4

# gpg: Signature made Fri 07 Apr 2017 13:44:17 BST
# gpg:                using RSA key 0x7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
  mirror: Fix aio context of mirror_top_bs
  block: Assert attached child node has right aio context
  block: Fix unpaired aio_disable_external in external snapshot
  block: Don't check permissions for copy on read
  qemu-img: img_create does not support image-opts, fix docs
  iotests: Add mirror tests for orphaned source
  block/mirror: Fix use-after-free
  commit: Set commit_top_bs->total_sectors
  commit: Set commit_top_bs->aio_context
  block: Ignore guest dev permissions during incoming migration

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