'generate a vdi_map for _call_live_migrate_command.'

def _generate_vdi_map(self, destination_sr_ref, vm_ref, sr_ref=None):

    if (sr_ref is None):
        sr_ref = vm_utils.safe_find_sr(self._session)
    vm_vdis = vm_utils.get_instance_vdis_for_sr(self._session, vm_ref, sr_ref)
    return {vdi: destination_sr_ref for vdi in vm_vdis}
