Use python discovered through env instead of hardcoding a path

This is more flexible regarding the location of the python binary
but doesn't allow to pass the -u flag. The -i flag can be passed
from inside the script using the PYTHONINSPECT env variable.

This fixes a problem with the esx_vi_generator.py on FreeBSD.

examples/python/dominfo.py

@@ -1,4 +1,4 @@
-#! /usr/bin/python
+#!/usr/bin/env python
 # dominfo - print some information about a domain
 
 import libvirt