net: L2TPv3 transport

net: L2TPv3 transport

This transport allows to connect a QEMU nic to a static Ethernet
over L2TPv3 tunnel. The transport supports all options present
in the Linux kernel implementation. It allows QEMU to connect
to any Linux host running kernel 3.3+, most routers and network
devices as well as other QEMU instances.

[Fixed up net_client_init1() switch statement to support -netdev
--Stefan]

Signed-off-by: Anton Ivanov <antivano@cisco.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>