[COMMANDS] cmd_host: Add sub-commands to list buses, classes, and devices

This patch adds following sub-commands to host commands:
bus_list          - List all buses
bus_device_list   - List all devices on given bus
class_list        - List all classes
class_device_list - List all devices on given class

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