Enable virDomainMemoryStats in the python API.  dom.memoryStats() will return a
dictionary containing the supported statistics.  A dictionary is required
because the meaining of each quantity cannot be inferred from its index in a
list.

* python/generator.py: reenable bindings for this entry point
* python/libvirt-override-api.xml python/libvirt-override.c: the
  generator can't handle this new function, add the new binding,
  and the XML description