oft: Enclose 'xmlrunner' import in try-except

If the 'xmlrunner' package cannot be imported, an exception is
thrown. Unhandled, the datapath will not shut down causing the
program to hang. Ensure the datapath is killed before re-raising
the exception.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>
diff --git a/oft b/oft
index f6897bb..d89ef3a 100755
--- a/oft
+++ b/oft
@@ -573,8 +573,12 @@
 
     logging.info("*** TEST RUN START: " + time.asctime())
     if config["xunit"]:
-        import xmlrunner  # fail-fast if module missing
-
+        try:
+            import xmlrunner  # fail-fast if module missing
+        except ImportError as ex:
+            oftest.dataplane_instance.kill()
+            profiler_teardown(profiler)
+            raise ex
         runner = xmlrunner.XMLTestRunner(output=config["xunit_dir"],
                                          outsuffix="",
                                          verbosity=2)