vmdk: Use vmdk_find_index_in_cluster everywhere

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 90df601f)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>