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 <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210116214705.822267-14-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>