Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2020-05-13' into staging

Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2020-05-13' into staging

Block patches:
- zstd compression for qcow2
- Fix use-after-free

# gpg: Signature made Wed 13 May 2020 15:14:06 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-2020-05-13:
  block/block-copy: fix use-after-free of task pointer
  iotests: 287: add qcow2 compression type test
  qcow2: add zstd cluster compression
  qcow2: rework the cluster compression routine
  qcow2: introduce compression type feature

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