VPP-95 VXLAN add_del API intermittent failure

VPP-95 VXLAN add_del API intermittent failure

The VXLAN add_del API sometimes rejects calls with
VNET_API_ERROR_SAME_SRC_DST when the two parameters are obviously
different. This is because the API code is checking the "is_ip6" flag in
a not-yet initialized area of memory leading to random results.

Also fix help text for vxlan_add_del_tunnel in vat.

This does not change the API; it repairs intermittent failure.

Change-Id: I41863694f478c1f669078f82f69d8cca4841c989
Signed-off-by: Chris Luke <chrisy@flirble.org>