usb: add serial bus property

This patch adds a serial property for all usb devices, which can be
used to set the serial number of a usb device (as listed by lsusb -v)
to a specific value.  Applies to emulated devices only.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/include/hw/usb.h b/include/hw/usb.h
index 4d9d05e..901b0da 100644
--- a/include/hw/usb.h
+++ b/include/hw/usb.h
@@ -205,6 +205,7 @@
     DeviceState qdev;
     USBPort *port;
     char *port_path;
+    char *serial;
     void *opaque;
     uint32_t flags;