python: Stop using xrange().

python: Stop using xrange().

Python 2 had range() and xrange().  xrange() is more efficient, but
behaves differently so range() was retained for compatibility.  Python 3
only has range() and it behaves like Python 2's xrange().

Remove explicit use of xrange() and use six.moves.range() to
make sure we're using xrange() from Python 2 or range() from Python 3.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>