A couple places in the Xen code have been using syntax that is
python2-specific. This replaces those instances with code that will
work in both python2 and python3.
PEP 3108 [1] moves urllib.urlopen to urllib.request.urlopen. The six
module is used in order to work in both python2 and python3 [2].
PEP 3110 [3] changed the grammar for except clauses, such that the
'as' keyword is now required in place of a comma when specifying an
exception variable.
[1] https://www.python.org/dev/peps/pep-3108/
[2] https://pythonhosted.org/six/#module-six.moves.urllib.request
[3] https://www.python.org/dev/peps/pep-3110/
Change-Id: I1235d767718a4207f4cef3e5b140319d003ad7b0
d7ed9a550b