usb_device_check:add max_devices_under_one_hub

It checks 8 usb mouses attached to a usb hub.
Signed-off-by: NHaotong Chen <hachen@redhat.com>

qemu/tests/cfg/usb_device_check.cfg

@@ -85,3 +85,6 @@
         - input_devices_under_two_tier_hub:
             no usb-ehci
             usb_topology = '{"usb-hub":2,"usb-mouse":1,"usb-kbd":1,"usb-tablet":1}'
+        - max_devices_under_one_hub:
+            no usb-ehci
+            usb_topology = '{"usb-hub":1,"usb-mouse":8}'