hotplug_mem: Fix undefined name 'exceptions'

Signed-off-by: NWei Jiangang <weijg.fnst@cn.fujitsu.com>

qemu/tests/hotplug_mem.py

@@ -3,6 +3,7 @@ import logging
 from virttest.utils_test import BackgroundTest
 from virttest.utils_test import run_virt_sub_test
 from virttest.utils_test.qemu import MemoryHotplugTest
+from avocado.core import exceptions
 
 # Make it work under both autotest-framework and avocado-framework
 try: