qa: fix setting rbd_sparse_read_threshold_bytes in test_migration_clone()

qa: fix setting rbd_sparse_read_threshold_bytes in test_migration_clone()

Currently it's set on the intermediary clone instead of the parent.
As a result the setting is effective only for reads that terminate at
the intermediary clone -- reads that go all the way to the parent may
end up being handled as not sparse depending on their size.

Fixes: https://tracker.ceph.com/issues/76101
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>