Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2021-09-27-v2' into staging

nbd patches for 2021-09-27

- Vladimir Sementsov-Ogievskiy: Rework coroutines of qemu NBD client
  to improve reconnect support
- Eric Blake: Relax server in regards to NBD_OPT_LIST_META_CONTEXT
- Vladimir Sementsov-Ogievskiy: Plumb up 64-bit bulk-zeroing support
  in block layer, in preparation for future NBD spec extensions
- Nir Soffer: Default to writeback cache in qemu-nbd

# gpg: Signature made Wed 29 Sep 2021 22:07:58 BST
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2021-09-27-v2:
  block/nbd: check that received handle is valid
  block/nbd: drop connection_co
  block/nbd: refactor nbd_recv_coroutines_wake_all()
  block/nbd: move nbd_recv_coroutines_wake_all() up
  block/nbd: nbd_channel_error() shutdown channel unconditionally
  nbd/client-connection: nbd_co_establish_connection(): fix non set errp
  nbd/server: Allow LIST_META_CONTEXT without STRUCTURED_REPLY
  block/io: allow 64bit discard requests
  block: use int64_t instead of int in driver discard handlers
  block: make BlockLimits::max_pdiscard 64bit
  block/io: allow 64bit write-zeroes requests
  block: use int64_t instead of int in driver write_zeroes handlers
  block: make BlockLimits::max_pwrite_zeroes 64bit
  block: use int64_t instead of uint64_t in copy_range driver handlers
  block: use int64_t instead of uint64_t in driver write handlers
  block: use int64_t instead of uint64_t in driver read handlers
  qcow2: check request on vmstate save/load path
  block/io: bring request check to bdrv_co_(read,write)v_vmstate
  qemu-nbd: Change default cache mode to writeback

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