During test in a cross-hypervisor environment (KVM+PowerVM), user will run into an issue during deploying an instance for Power host by PowerVM driver, though the property "hypervisor_type" for the PowerVM image have been configured. Since the _update_host_stats of PowerVM driver missing the attribute "supported_instances". Fixes bug: 1155498 Change-Id: I81259d153a7b5ae35747e57c53b14e0075d28d78