Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2019-05-28' into staging

Block patches:
- qcow2: Use threads for encrypted I/O
- qemu-img rebase: Optimizations
- backup job: Allow any source node, and some refactoring
- Some general simplifications in the block layer

# gpg: Signature made Tue 28 May 2019 20:26:56 BST
# gpg:                using RSA key 91BEB60A30DB3E8857D11829F407DB0061D5CF40
# gpg:                issuer "mreitz@redhat.com"
# gpg: Good signature from "Max Reitz <mreitz@redhat.com>" [full]
# Primary key fingerprint: 91BE B60A 30DB 3E88 57D1  1829 F407 DB00 61D5 CF40

* remotes/maxreitz/tags/pull-block-2019-05-28: (21 commits)
  blockdev: loosen restrictions on drive-backup source node
  qcow2-bitmap: initialize bitmap directory alignment
  qcow2: skip writing zero buffers to empty COW areas
  qemu-img: rebase: Reuse in-chain BlockDriverState
  qemu-img: rebase: Reduce reads on in-chain rebase
  qemu-img: rebase: Reuse parent BlockDriverState
  block: Make bdrv_root_attach_child() unref child_bs on failure
  block: Use bdrv_unref_child() for all children in bdrv_close()
  block/backup: refactor: split out backup_calculate_cluster_size
  block/backup: unify different modes code path
  block/backup: refactor and tolerate unallocated cluster skipping
  block/backup: move to copy_bitmap with granularity
  block/backup: simplify backup_incremental_init_copy_bitmap
  qcow2: do encryption in threads
  qcow2: bdrv_co_pwritev: move encryption code out of the lock
  qcow2: qcow2_co_preadv: improve locking
  qcow2-threads: split out generic path
  qcow2-threads: qcow2_co_do_compress: protect queuing by mutex
  qcow2-threads: use thread_pool_submit_co
  qcow2: add separate file for threaded data processing functions
  ...

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