iotests: 219: prepare for backup over block-copy

iotests: 219: prepare for backup over block-copy

The further change of moving backup to be a one block-copy call will
make copying chunk-size and cluster-size two separate things. So, even
with 64k cluster sized qcow2 image, default chunk would be 1M.
Test 219 depends on specified chunk-size. Update it for explicit
chunk-size for backup as for mirror.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-Id: <20210116214705[email protected]>
Signed-off-by: Max Reitz <[email protected]>