usb: separate out legacy usb registration from type registration

Type registeration is going to get turned into a QOM call so decouple the
legacy support.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/usb-serial.c b/hw/usb-serial.c
index de49607..00b4985 100644
--- a/hw/usb-serial.c
+++ b/hw/usb-serial.c
@@ -622,7 +622,9 @@
 
 static void usb_serial_register_devices(void)
 {
-    usb_qdev_register(&serial_info, "serial", usb_serial_init);
-    usb_qdev_register(&braille_info, "braille", usb_braille_init);
+    usb_qdev_register(&serial_info);
+    usb_legacy_register("usb-serial", "serial", usb_serial_init);
+    usb_qdev_register(&braille_info);
+    usb_legacy_register("usb-braille", "braille", usb_braille_init);
 }
 device_init(usb_serial_register_devices)