qmp: netdev_add is like -netdev, not -net, fix documentation Cc: qemu-stable@nongnu.org Signed-off-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Laszlo Ersek <lersek@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>