Merge remote-tracking branch 'stefanha/block' into staging

Merge remote-tracking branch 'stefanha/block' into staging

# By Stefan Hajnoczi (14) and others
# Via Stefan Hajnoczi
* stefanha/block: (28 commits)
  blockdev: Fix up copyright and permission notice
  qemu-iotests: use -nographic in test case 007
  qemu-iotests: add tests for rebasing zero clusters
  dataplane: fix hang introduced by AioContext transition
  coroutine: use AioContext for CoQueue BH
  threadpool: drop global thread pool
  block: add bdrv_get_aio_context()
  aio: add a ThreadPool instance to AioContext
  threadpool: add thread_pool_new() and thread_pool_free()
  threadpool: move globals into struct ThreadPool
  main-loop: add qemu_get_aio_context()
  sheepdog: set io_flush handler in do_co_req
  sheepdog: use non-blocking fd in coroutine context
  qcow2: make is_allocated return true for zero clusters
  qcow2: drop unnecessary flush in qcow2_update_snapshot_refcount()
  qcow2: drop flush in update_cluster_refcount()
  qcow2: flush in qcow2_update_snapshot_refcount()
  qcow2: set L2 cache dependency in qcow2_alloc_bytes()
  qcow2: flush refcount cache correctly in qcow2_write_snapshots()
  qcow2: flush refcount cache correctly in alloc_refcount_block()
  ...