fix dummy security_policy_dir

Changeset 15951:ad339d88639d split policy_dir_prefix into
security_dir_prefix and policy_dir_prefix in acm code. I think this needs to be reflected in the dummy module as well (otherwise we blowup when bootloader.py tries to reference security.security_dir_prefix). Fixed in below patch. Thanks,

Alex

---

diff -r 9426c74fa6e2 tools/python/xen/util/xsm/dummy/dummy.py
@@ -6,6 +6,7 @@ class XSMError(Exception):
     def __str__(self):
         return repr(self.value)
 
+security_dir_prefix = "";
 policy_dir_prefix = "";
 active_policy = "";
 NULL_SSIDREF = 0;