cluster_size got from backup_calculate_cluster_size()

[Derek Su]

Hi,

The cluster_size got from backup_calculate_cluster_size(),
MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size), is 64K regardless
of the target image's cluster size.

For example:

If the cluster size of source and target qcow2 images are both 16K, the 
64K from backup_calculate_cluster_size() results in unwanted copies of 
clusters.

The behavior slows down the backup (block-copy) process when the
source image receives lots of rand writes.


Is the following calculation reasonable for the above issue?


```
static int64_t backup_calculate_cluster_size(BlockDriverState *target,
                                             Error **errp)
{
    ...

    ret = bdrv_get_info(target, &bdi);

    ...

    return (bdi.cluster_size == 0 ?
                BACKUP_CLUSTER_SIZE_DEFAULT : cluster_size);
}

```

Thanks.
Regards,

Derek