vmdk: Fix partial overwrite of zero cluster

When overwriting a zero cluster, we must not perform copy-on-write from
the backing file, but from a zeroed buffer.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200430133007.170335-4-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>