vmdk: Fix integer overflow in offset calculation

This fixes the bug introduced by commit c6ac36e (vmdk: Optimize cluster
allocation).

$ ~/build/master/qemu-io /stor/vm/arch.vmdk -c 'write 2G 1k'
write failed: Invalid argument

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1411437381-11234-1-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
