python: Expose virDomain{G,S}etInterfaceParameters APIs in python binding

The v4 patch corrects indentation issues.

The v3 patch follows latest python binding codes and change 'size'
type from int to Py_ssize_t.

An simple example to show how to use it:

#!/usr/bin/env python

import libvirt

conn = libvirt.open(None)
dom = conn.lookupByName('foo')

print dom.interfaceParameters('vnet0', 0)

params = {'outbound.peak': 10,
          'inbound.peak': 10,
          'inbound.burst': 20,
          'inbound.average': 20,
          'outbound.average': 30,
          'outbound.burst': 30}

print dom.setInterfaceParameters('vnet0', params, 0)
print dom.interfaceParameters('vnet0', 0)

Signed-off-by: Alex Jia <ajia@redhat.com>