Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2020-03-24' into staging

Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2020-03-24' into staging

Block patches for 5.0-rc0:
- Use-after-free fix
- Fix for a memleak in an error path
- Preventative measures against other potential use-after-frees, and
  against NULL deferences at runtime
- iotest fixes

# gpg: Signature made Tue 24 Mar 2020 12:19:09 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-2020-03-24:
  iotests/026: Move v3-exclusive test to new file
  iotests: Fix cleanup path in some tests
  block/qcow2: zero data_file child after free
  block: bdrv_set_backing_bs: fix use-after-free
  block: Assert BlockDriver::format_name is not NULL
  block: Avoid memleak on qcow2 image info failure

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