vmdk: Fix index_in_cluster calculation in vmdk_co_get_block_status

It has the similar issue with b1649fae. Since the calculation
is repeated for a few times already, introduce a function so it can be
reused.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>