[CORE] vmm_devdrv: Remove few redundant APIs

This patch does minor simplification to vmm_devdrv API by removing
few API such as vmm_devdrv_class_register/unregister_device() and
and vmm_devdrv_bus_register/unregister_device() from vmm_devdrv.h

This means that now we use vmm_devdrv_register/unregister_device()
APIs to support bus/class level device registration.

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