Description of problem:

Vdsm 4.40.32 added support for downloading single volume from a volume chain
by specifying new backing_chain argument. Engine 4.4.3.7 or later always
specify backing_chain=False or backing_chain=True.

When using newer vdsm >= 4.4.32 with older engine < 4.4.3.7 that does not
specify the backing_chain argument, the NBD server is configured with
backing_chain=None, which is treated as backing_chain=False instead of backing_chain=True.

Users downloading images with multiple snapshots will get the only the data
of the last snapshot.

Users downloading specific snapshot will get only the sepcidied snapshot
instead of the entire chain starting at the snapshot.

Here is example commands showing the wrong NBD server configuration:

Original chain:

# qemu-img info --backing-chain /rhev/data-center/mnt/nfs1\:_export_3/f5915245-0ac5-4712-b8b2-dd4d4be7cdc4/images/4b62aa6d-3bdd-4db3-b26f-0484c4124631/0a44d697-41bd-4f38-81f2-e955f51799c4
image: /rhev/data-center/mnt/nfs1:_export_3/f5915245-0ac5-4712-b8b2-dd4d4be7cdc4/images/4b62aa6d-3bdd-4db3-b26f-0484c4124631/0a44d697-41bd-4f38-81f2-e955f51799c4
file format: qcow2
virtual size: 6 GiB (6442450944 bytes)
disk size: 72.4 MiB
cluster_size: 65536
backing file: 20391fbc-fa77-4fef-9aea-cf59f27f90b5 (actual path: /rhev/data-center/mnt/nfs1:_export_3/f5915245-0ac5-4712-b8b2-dd4d4be7cdc4/images/4b62aa6d-3bdd-4db3-b26f-0484c4124631/20391fbc-fa77-4fef-9aea-cf59f27f90b5)
backing file format: qcow2
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

image: /rhev/data-center/mnt/nfs1:_export_3/f5915245-0ac5-4712-b8b2-dd4d4be7cdc4/images/4b62aa6d-3bdd-4db3-b26f-0484c4124631/20391fbc-fa77-4fef-9aea-cf59f27f90b5
file format: qcow2
virtual size: 6 GiB (6442450944 bytes)
disk size: 1.65 GiB
cluster_size: 65536
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

Starting NBD server:

# cat nbd.json 
{
  "server_id": "test",
  "config": {
    "sd_id": "f5915245-0ac5-4712-b8b2-dd4d4be7cdc4",
      "img_id": "4b62aa6d-3bdd-4db3-b26f-0484c4124631",
      "vol_id": "0a44d697-41bd-4f38-81f2-e955f51799c4",
      "readonly": true
  }
}

# vdsm-client -f nbd.json NBD start_server
"nbd:unix:/run/vdsm/nbd/test.sock"

The qemu-nbd process:

# ps -ef | grep qemu-nbd | grep -v grep
vdsm      252402       1  0 18:49 ?        00:00:00 /usr/bin/qemu-nbd --socket /run/vdsm/nbd/test.sock --persistent --shared=8 --export-name= --cache=none --aio=native --read-only json:{"driver": "qcow2", "file": {"driver": "file", "filename": "/rhev/data-center/mnt/nfs1:_export_3/f5915245-0ac5-4712-b8b2-dd4d4be7cdc4/images/4b62aa6d-3bdd-4db3-b26f-0484c4124631/0a44d697-41bd-4f38-81f2-e955f51799c4"}, "backing": null}

Note: "backing": null - the server export only the top volume.
(this is the root cause).

Downloading the image:

# qemu-img convert -f raw -O qcow2 nbd:unix:/run/vdsm/nbd/test.sock download.qcow2

The downloaded image:

# qemu-img info download.qcow2 
image: download.qcow2
file format: qcow2
virtual size: 6 GiB (6442450944 bytes)
disk size: 42.9 MiB
cluster_size: 65536
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

The download includes only data from the top volume, instead of
the entire chain.

Version-Release number of selected component (if applicable):
v4.40.32

How reproducible:
Always

Steps to Reproduce:
1. Use engine < 4.4.3.7 with newer vdsm >= 4.40.32
2. Create VM with one disk with some data
3. Stop the VM
4. Create a snapshot including the disk
4. Download the disk using download_disk.py

Actual results:
The download includes only the snapshot. Since the snapshot is empty,
the download will be empty image.

Expected results:
the download includes the entire disk.