hotplug_mem: Increase sub_test_wait_time

Increase sub_test_wait_time to ensure memory device hotplug/uplug when VM
is paused.

Signed-off-by: Wang Ziming <wangziming.huawei.com>

qemu/tests/cfg/hotplug_mem.cfg

@@ -148,7 +148,7 @@
             sub_type = stop_continue
             pause_time = 300
             wait_resume_timeout = "${pause_time}"
-            sub_test_wait_time = 60
+            sub_test_wait_time = 80
         - stress:
             sub_test_wait_time = 60
             Windows: