optional_plugins.runner_vm: Import missing module

The `exceptions` module is not imported even though it's suppose to be
used.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

optional_plugins/runner_vm/avocado_runner_vm/__init__.py

@@ -21,7 +21,7 @@ from xml.dom import minidom
 
 import libvirt
 
-from avocado.core import exit_codes
+from avocado.core import exit_codes, exceptions
 from avocado.core.plugin_interfaces import CLI
 from avocado_runner_remote import Remote, RemoteTestRunner