vhost: support vhost-user

In rte_vhost_driver_register(), vhost unix domain socket listener fd is created
and added to polled(based on select) fdset.

In rte_vhost_driver_session_start(), fds in the fdset are checked for
processing. If there is new connection from qemu, connection fd accepted is
added to polled fdset. The listener and connection fds in the fdset are
then both checked. When there is message on the connection fd, its
callback vserver_message_handler is called to process vhost-user messages.

To support identifying which virtio is from which guest VM, we could call
rte_vhost_driver_register with different socket path. Virtio devices from
same VM will connect to VM specific socket. The socket path information is
stored in the virtio_net structure.

Signed-off-by: Huawei Xie <huawei.xie@intel.com>
Signed-off-by: Przemyslaw Czesnowicz <przemyslaw.czesnowicz@intel.com>
Acked-by: Tetsuya Mukawa <mukawa@igel.co.jp>