vsock: add local transport support in the vsock core

This patch allows to register a transport able to handle
local communication (loopback).

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>