usb-bus: convert USBDeviceClass init to realize

Add "realize/unrealize" in USBDeviceClass, which has errp
as a parameter. So all the implementations now use
error_setg instead of error_report for reporting error.

Note: this patch still keep "init" in USBDeviceClass, and
call kclass->init in usb_device_realize(), avoid breaking
git bisect. After realize all usb devices, will be removed.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
