Extend IPv6 subnets to /64 if network_size is set smaller than /64

If cidr_v6 is set with network_size smaller than /64, this fix
extends the subnet to /64. Fixes bug #1011122.

Radvd (which is used by nova-network) also requires as /64 for
autoconf at the client(instance) side.

Change-Id: I0a6ac363e46dd9b8c9cfb750ce37720fdbc56a1f

nova/network/manager.py

@@ -1508,6 +1508,9 @@ def _do_create_networks(self, context,
             if cidr_v6:
                 fixed_net_v6 = netaddr.IPNetwork(cidr_v6)
                 prefixlen_v6 = 128 - subnet_bits
+                # smallest subnet in IPv6 ethernet network is /64
+                if prefixlen_v6 > 64:
+                    prefixlen_v6 = 64
                 subnets_v6 = fixed_net_v6.subnet(prefixlen_v6,
                                                  count=num_networks)
             if cidr: