Fix xenapi functions' tests

The tests got outdated, this fix makes the tests pass again.

Change-Id: Iadddfbf34bf79ba455811645e766c2f3d0fcca84
diff --git a/tools/xen/mocks b/tools/xen/mocks
index 94b0ca4..ec8679e 100644
--- a/tools/xen/mocks
+++ b/tools/xen/mocks
@@ -73,7 +73,7 @@
         done
         return 1
     fi
-    echo "Mock test does not implement the requested function"
+    echo "Mock test does not implement the requested function: ${1:-}"
     exit 1
 }