qcow2: Reuse preallocated zero clusters

qcow2: Reuse preallocated zero clusters

Instead of just freeing preallocated zero clusters and completely
allocating them from scratch, reuse them.

We cannot do this in handle_copied(), however, since this is a COW
operation. Therefore, we have to add the new logic to handle_alloc() and
simply return the existing offset if it exists. The only catch is that
we have to convince qcow2_alloc_cluster_link_l2() not to free the old
clusters (because we have reused them).

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>