[DRIVERS] usb: Use struct vmm_device_type to distinguish devices on usb bus

Both usb_device and usb_interface are registered with vmm_devdrv as
vmm_device.

This patch makes use of struct vmm_device_type to distinguish between
usb_device and usb_interface.

Signed-off-by: Anup Patel <anup@brainfault.org>