extra-tests: Add suppression for libvirt/gnutls leak.

https://www.redhat.com/archives/libvir-list/2012-April/msg00322.html

tests/extra/suppressions

@@ -164,4 +164,15 @@
    Memcheck:Cond
    fun:index
    fun:expand_dynamic_string_token
-}
+}
+
+# libvirt calls gnutls_global_init but doesn't call the
+# corresponding gnutls_global_deinit function because of
+# thread-safety issues (in GnuTLS).
+{
+   libvirt_leak
+   Memcheck:Leak
+   ...
+   fun:gnutls_global_init
+   fun:virInitialize
+}