This patchs adds infrastructure to handle -usbdevice via qdev callbacks.
USBDeviceInfo gets a name field (for the -usbdevice driver name) and a
callback for -usbdevice parameter parsing.

The new usbdevice_create() function walks the qdev driver list and looks
for a usb driver with a matching name.  When a parameter parsing
callback is present it is called, otherwise the device is created via
usb_create_simple().
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>