Now DPDK vhost lib has been generic enough, that it can be used to implement any vhost-user drivers. For example, this patch implements a very simple vhost-user net driver, mainly for demonstrating how to use those generic vhost APIs. And when the --builtin-net-driver option is used, the example virtio-net driver code will be invoked, instead of the one provided from the vhost library. Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>