examples/vhost: demonstrate the new generic APIs

examples/vhost: demonstrate the new generic APIs

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>