Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer patches

# gpg: Signature made Fri 23 Oct 2015 17:59:56 BST using RSA key ID C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"

* remotes/kevin/tags/for-upstream: (37 commits)
  tests: Add test case for aio_disable_external
  block: Add "drained begin/end" for internal snapshot
  block: Add "drained begin/end" for transactional blockdev-backup
  block: Add "drained begin/end" for transactional backup
  block: Add "drained begin/end" for transactional external snapshot
  block: Introduce "drained begin/end" API
  aio: introduce aio_{disable,enable}_external
  dataplane: Mark host notifiers' client type as "external"
  nbd: Mark fd handlers client type as "external"
  aio: Add "is_external" flag for event handlers
  throttle: Remove throttle_group_lock/unlock()
  blockdev: Allow more options for BB-less BDS tree
  blockdev: Pull out blockdev option extraction
  blockdev: Do not create BDS for empty drive
  block: Prepare for NULL BDS
  block: Add blk_insert_bs()
  block: Prepare remaining BB functions for NULL BDS
  block: Fail requests to empty BlockBackend
  block: Make some BB functions fall back to BBRS
  block: Add BlockBackendRootState
  ...

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