rasd:Parent in add_disk could be different than '1'

For now we try to reuse an existing value and raise an error when:
- a valid disk doesn't exist
- different disks exist with different rasd:Parent

The official documentation doesn't really help here:
http://pubs.vmware.com/vcd-55/index.jsp#com.vmware.vcloud.api.doc_55/GUID-E56EEEEA-EDED-4EBC-8095-08AD2668F0D2.html