zebra: data structure changes for single vxlan device

zebra: data structure changes for single vxlan device

This changeset introduces the data structure changes needed for
single vxlan device functionality. A new struct zebra_vxlan_vni_info
encodes the iftype and vni information for vxlan device.

The change addresses related access changes of the new data structure
fields from different files

zebra_vty is modified to take care of the vni dump information according
to the new vni data structure for vxlan devices.

Signed-off-by: Sharath Ramamurthy <sramamurthy@nvidia.com>