capability_xml: Add getter for power_management

Add a function to get a list contains all supported power management
methods.

Signed-off-by: Hao Liu <hliu@redhat.com>
Ah, okey. It just display the supported methods as a message. It is readonly. But I think we should add a 
```
accessors.AllForbidden(property_name="power_management_list",
                               libvirtxml=self)
```
to forbid getter and setter.