Add public API for getting migration speed

Includes impl of python binding since the generator was not
able to cope.

Note: Requires gendispatch.pl patch from Matthias Bolte

https://www.redhat.com/archives/libvir-list/2011-August/msg01367.html

generator.py

@@ -372,6 +372,7 @@ skip_impl = (
     'virNodeGetCPUStats',
     'virNodeGetMemoryStats',
     'virDomainGetBlockJobInfo',
+    'virDomainMigrateGetMaxSpeed',
 )