Implement ovirt_vm_disk_attachments data source

[ghost]

Please describe what you would like to see

Implement a ovirt_vm_disk_attachments data source that returns all disk attachments for a VM.

Please describe the solution you'd like

The data source should look like this:

data "ovirt_vm_disk_attachments" "foo" {
  vm_id = var.vm_id
}

The result should contain the following fields:

• disk_id

Optionally, it may contain all additional fields from the DiskAttachment objects from go-ovirt-client.

Please describe your use case

We need to implement a resource that resizes all disks inherited from the template after VM creation. See openshift/installer#5867 for details.