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

Block layer patches:

- Add block export infrastructure
- iotests improvements
- Document the throttle block filter
- Misc code cleanups

# gpg: Signature made Fri 02 Oct 2020 15:36:53 BST
# gpg:                using RSA key DC3DEB159A9AF95D3D7456FE7F09B272C88F2FD6
# gpg:                issuer "kwolf@redhat.com"
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>" [full]
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream: (37 commits)
  qcow2: Use L1E_SIZE in qcow2_write_l1_entry()
  qemu-storage-daemon: Fix help line for --export
  iotests: Test block-export-* QMP interface
  iotests: Allow supported and unsupported formats at the same time
  iotests: Introduce qemu_nbd_list_log()
  iotests: Factor out qemu_tool_pipe_and_status()
  nbd: Deprecate nbd-server-add/remove
  nbd: Merge nbd_export_new() and nbd_export_create()
  block/export: Move writable to BlockExportOptions
  block/export: Add query-block-exports
  block/export: Create BlockBackend in blk_exp_add()
  block/export: Move blk to BlockExport
  block/export: Add BLOCK_EXPORT_DELETED event
  block/export: Add block-export-del
  block/export: Move strong user reference to block_exports
  block/export: Add 'id' option to block-export-add
  block/export: Add blk_exp_close_all(_type)
  block/export: Allocate BlockExport in blk_exp_add()
  block/export: Add node-name to BlockExportOptions
  block/export: Move AioContext from NBDExport to BlockExport
  ...

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