resource/ovirt_disk_attachment: testAccCheckDiskAttachmentDestroy function won't work as expected

[imjoey]

There are two bugs in current testAccCheckDiskAttachmentDestroy function:

1. It's using rs.Primary.ID as the disk ID to fetch a disk through oVirt engine API. But in this case, the rs.Primary.ID is combined with vmID and diskID, in the format of df736600-b8be-4029-be98-4b0611be6be4:5bd12e84-025a-0171-03aa-0000000003d6, so the code logic will always treat this resource as already removed, regardless of whether it exists in reality.
2. Actually, the destroy of a disk attachment indicates it has been detached from a VM and the relevant disk probably still exist in oVirt. While current code uses the result of the existence of the disk and it will surely be incorrect.