Image formats that grow the image file on demand and are organized into
clusters must handle sub-cluster allocating writes.  Such writes touch
a portion of a previously unallocated data cluster.  After the image
file is grown with the written data, reads of that cluster should work
as expected:

1. Sectors before the written region are zero.
2. The written region is present and the data is uncorrupted.
3. Sectors after the written region are zero.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>