Hello,
api.vms.get(VM_NAME).disks.add(params.Disk(storage_domains=params.StorageDomains(storage_domain=[api.storagedomains.get(STORAGE_NAME)]),
size=512*MB,
# type_='system', - disk type is deprecated
status=None,
interface='virtio',
format='cow',
sparse=True,
bootable=True))