Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2020-01-06' into staging

Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2020-01-06' into staging

Block patches:
- Minor fixes and tests from the freeze period (too minor to be included
  in 4.2)
- Allow many bash iotests to test qcow2's external data file feature
- Add compress filter driver
- Fix Python iotests after 6f6e1698a6
- Fix for the backup job

# gpg: Signature made Mon 06 Jan 2020 14:33:06 GMT
# 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-2020-01-06: (34 commits)
  backup-top: Begin drain earlier
  tests/qemu-iotests: Update tests to recent desugarized -accel option
  tests/qemu-iotests: add case to write compressed data of multiple clusters
  qcow2: Allow writing compressed data of multiple clusters
  block: introduce compress filter driver
  iotests: Allow check -o data_file
  iotests: Disable data_file where it cannot be used
  iotests: Make 198 work with data_file
  iotests: Make 137 work with data_file
  iotests: Make 110 work with data_file
  iotests: Make 091 work with data_file
  iotests: Avoid cp/mv of test images
  iotests: Use _rm_test_img for deleting test images
  iotests: Avoid qemu-img create
  iotests: Drop IMGOPTS use in 267
  iotests: Replace IMGOPTS='' by --no-opts
  iotests: Replace IMGOPTS= by -o
  iotests: Inject space into -ocompat=0.10 in 051
  iotests: Add -o and --no-opts to _make_test_img
  iotests: Let _make_test_img parse its parameters
  ...

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