Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2020-08-26' into staging

Block patches:
- qcow2 subclusters (extended L2 entries)

# gpg: Signature made Wed 26 Aug 2020 08:37:04 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-2020-08-26: (34 commits)
  iotests: Add tests for qcow2 images with extended L2 entries
  qcow2: Assert that expand_zero_clusters_in_l1() does not support subclusters
  qcow2: Allow preallocation and backing files if extended_l2 is set
  qcow2: Add the 'extended_l2' option and the QCOW2_INCOMPAT_EXTL2 bit
  qcow2: Add prealloc field to QCowL2Meta
  qcow2: Add subcluster support to qcow2_measure()
  qcow2: Add subcluster support to qcow2_co_pwrite_zeroes()
  qcow2: Add subcluster support to handle_alloc_space()
  qcow2: Clear the L2 bitmap when allocating a compressed cluster
  qcow2: Update L2 bitmap in qcow2_alloc_cluster_link_l2()
  qcow2: Add subcluster support to check_refcounts_l2()
  qcow2: Add subcluster support to discard_in_l2_slice()
  qcow2: Add subcluster support to zero_in_l2_slice()
  qcow2: Add subcluster support to qcow2_get_host_offset()
  qcow2: Add subcluster support to calculate_l2_meta()
  qcow2: Handle QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC
  qcow2: Replace QCOW2_CLUSTER_* with QCOW2_SUBCLUSTER_*
  qcow2: Add cluster type parameter to qcow2_get_host_offset()
  qcow2: Add qcow2_cluster_is_allocated()
  qcow2: Add qcow2_get_subcluster_range_type()
  ...

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