Add jumbo frames support to non-dpdk vhost interfaces.

Add jumbo frames support to non-dpdk vhost interfaces.

This code provided inter-VM (2 cores per VM) throughput of
22Gbps using iperf through VPP (1 core) with 9k frames.
With the same setup and pktgen running on both sides, it
reached 5Mpps with no packets drop (Equivalent to before the patch).
During the tests the average vector length was about 1, which
likely means that VPP is not the bottleneck.

The patch also includes some generic functions for vlib buffers
allowing for chained buffer construction whether or not DPDK is enabled.

Change-Id: Icfd1803e84b2b4578f305ab730576211f6242d6a
Signed-off-by: Pierre Pfister <ppfister@cisco.com>