Cause: There was no validation on the storage_domain parameter on SDK / REST API. Therefore, it was "possible" to create a vm's disk_attachment without a storage_domain.
Consequence: Disk attachment has been created, and the storage_domain's parameter was set as the other VM's disk storage domain.
Fix: Add validation for the storage_domain parameter which supplied by the user (as a part of SDK / REST API command parameters).
Result: Error message which indicates that storage_domain wasn't supplied + operation stops.