vhost_net: use the function qemu_get_peer

user the qemu_get_peer to replace the old process

Signed-off-by: Cindy Lu <lulu@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20200701145538.22333-3-lulu@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>