Block layer patches:

- qcow2: Support for external data files
- qcow2: Default to 4KB for the qcow2 cache entry size
- Apply block driver whitelist for -drive format=help
- Several qemu-iotests improvements

# gpg: Signature made Fri 08 Mar 2019 12:54:27 GMT
# gpg:                using RSA key 7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>" [full]
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream: (33 commits)
  qcow2 spec: Describe string header extensions
  qemu-iotests: Add dependency to qemu-nbd tool
  ahci-test: Add dependency to qemu-img tool
  qemu-iotests: amend with external data file
  qemu-iotests: General tests for qcow2 with external data file
  qemu-iotests: Preallocation with external data file
  qcow2: Implement data-file-raw create option
  qcow2: Store data file name in the image
  qcow2: Creating images with external data file
  qcow2: Add basic data-file infrastructure
  qcow2: Support external data file in qemu-img check
  qcow2: Return error for snapshot operation with data file
  qcow2: External file I/O
  qcow2: Prepare qcow2_co_block_status() for data file
  qcow2: Return 0/-errno in qcow2_alloc_compressed_cluster_offset()
  qcow2: Don't assume 0 is an invalid cluster offset
  qcow2: Prepare count_contiguous_clusters() for external data file
  qcow2: Prepare qcow2_get_cluster_type() for external data file
  qcow2: Pass bs to qcow2_get_cluster_type()
  qcow2: Basic definitions for external data files
  ...
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>