VMDK: bugfix, align offset to cluster in get_whole_cluster

In get_whole_cluster, the offset is not aligned to cluster when reading
from backing_hd. When the first write to child is not at the cluster
boundary, wrong address data from parent is copied to child.

Signed-off-by: Fam Zheng <famcool@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>