CLOUDSTACK-4967: vxlan doesn't scale

- Fix inproper multicast address creation (when VNI > 65535)
- Fix missing bride name in delete oparation

Signed-off-by : Toshiaki Hatano <haeena@haeena.net>
Bugfix: Guest network isn't created correctly when vxlan is used for isolation method in rare case

Guest network isn't created correctly when vxlan is used for isolation method, and
physical network interface name length is longer than 4.
This commit fix this issue by raising error when long interface name is specified.