[analyzer] Add class for IPv6 HTTP server (NASA-SW-VnV#158).

This commit adds a new class HTTPServerIPv6 that inherits from
HTTPServer, but selects IPv6. In systems with dual stack (e.g., default
on linux), opening a server in IPv6 also listens in IPv4.

analyzer/python/ikos/http.py

@@ -50,3 +50,12 @@
     from urlparse import parse_qs
     from urllib import urlencode
     from urllib2 import urlopen
+
+
+class HTTPServerIPv6(HTTPServer):
+    '''
+    HTTP server that listens in IPv6.
+
+    In systems with dual stack, this also listens in IPv4.
+    '''
+    address_family = socket.AF_INET6