xen: Fix xen server crash in ping broadcast from vm

- Ping broadcast causes xen server to crash. The fix is to make network header
  offset calculation proper for 32bit host (xenserver).

Signed-off-by: Anirban Chakraborty <abchak@juniper.net>
Fix whitespace issue

Signed-off-by: Anirban Chakraborty <abchak@juniper.net>
Merge pull request #16 from anirban-c/master

xen: Fix for server crash