qemu.tests.cfg: Don't start vm in usb_multi_disk.max_disk test

The virt-test framework checks both 'start_vm' and 'restart_vm'
parameters before starting a VM. So if we don't want to start
VM by framework, we should set both of these parameters to 'no'.
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>

qemu/tests/cfg/usb.cfg

@@ -232,6 +232,7 @@
                     n_repeat = 10
                 - max_disk:
                     start_vm = no
+                    restart_vm = no
                     usbs += " usb2 usb3 usb4"
                     stg_image_num = 24
                     stg_image_size = 1G