Don't hardcode interpreter path

This is particularly useful on operating systems that don't ship
Python as part of the base system (eg. FreeBSD) while still working
just as well as it did before on Linux.

Reviewed-by: Daniel P. Berrange <berrange@redhat.com>

sanitytest.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 import sys
 import lxml