iotests: 185: prepare for backup over block-copy

iotests: 185: 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.
185 test however assumes, that with speed limited to 64K, one iteration
would result in offset=64K. It will change, as first iteration would
result in offset=1M independently of speed.

So, let's explicitly specify, what test wants: set max-chunk to 64K, so
that one iteration is 64K. Note, that we don't need to limit
max-workers, as block-copy rate limiter will handle the situation and
wouldn't start new workers when speed limit is obviously reached.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210116214705.822267-13-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>