Fixes PowerVM spawn failed as missing attr supported_instances

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
Merge "Fixes PowerVM spawn failed as missing attr supported_instances"