qcow2: Fix preallocation on images with unaligned sizes

qcow2: Fix preallocation on images with unaligned sizes

When resizing an image with qcow2_co_truncate() using the falloc or
full preallocation modes the code assumes that both the old and new
sizes are cluster-aligned.

There are two problems with this:

  1) The calculation of how many clusters are involved does not always
     get the right result.

     Example: creating a 60KB image and resizing it (with
     preallocation=full) to 80KB won't allocate the second cluster.

  2) No copy-on-write is performed, so in the previous example if
     there is a backing file then the first 60KB of the first cluster
     won't be filled with data from the backing file.

This patch fixes both issues.

Signed-off-by: Alberto Garcia <[email protected]>
Message-Id: <20200617140036[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Max Reitz <[email protected]>