import bpy
bpy.context.user_preferences.system.compute_device_type = 'CUDA'
bpy.context.user_preferences.system.compute_device = 'CUDA_MULTI_2'

AttributeError: 'UserPerferencesSystem' object has no attribute 'compute_device_type'

prefs = bpy.context.user_preferences.addons['cycles'].preferences
print(prefs.compute_device_type)

for d in prefs.devices:
    print(d.name)