Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2021-03-30' into staging

Block patches for 6.0-rc1:
- Mark the qcow2 cache clean timer as external to fix record/replay
- Fix the mirror filter node's permissions so that an external process
  cannot grab an image while it is used as the mirror source
- Add documentation about FUSE exports to the storage daemon
- When creating a qcow2 image with the data-file-raw option, all
  metadata structures should be preallocated
- iotest fixes

# gpg: Signature made Tue 30 Mar 2021 13:38:40 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-2021-03-30:
  iotests/244: Test preallocation for data-file-raw
  qcow2: Force preallocation with data-file-raw
  qsd: Document FUSE exports
  block/mirror: Fix mirror_top's permissions
  iotests/046: Filter request length
  qcow2: use external virtual timers
  iotests/116: Fix reference output
  iotests: fix 051.out expected output after error text touchups
  iotests: Fix typo in iotest 051

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