qemu.cfg.usb: Set max nest levels of hub device to 5.

Signed-off-by: Feng Yang <fyang@redhat.com>

ID: 1079874
Hi fyang,

As this is max usb hub test, it should be usb_port_hub* instead of usb_port_d*, right?
Or it can't find the hub device.

If I'm misunderstanding, please feel free to correct me.

Thanks.