ipfix-export: support sending of ipv6 pkts
ipfix-export: support sending of ipv6 pkts

The current code that sends ipfix packets assumes IPv4. Modify this
so that it generates and sends packets based on whether the destination
address is IPv4 or IPv6. Where code is common across multiple places
pull it out into helper functions.

Type: improvement
Signed-off-by: Paul Atkins <bd156abe841cbed4bdcc19e9b73d63734967d2c5@graphiant.com>
Change-Id: I216c6c4c1b58cacedca22019f74a38c64a368b7e

FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp