qcow2: Fail write_compressed when overwriting data

qcow2_alloc_compressed_cluster_offset() already fails if the copied flag
is set, because qcow2_write_compressed() doesn't perform COW as it would
have to do to allow this.

However, what we really want to check here is whether the cluster is
allocated or not. With internal snapshots the copied flag may not be set
on allocated clusters. Check the cluster offset instead.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
