Fix xmconfigtest

Commit ac63014c introduced a regression in the conversion of Xen
xm config to XML by emitting an empty <cmdline>.  Prior to this
commit, <cmdline> was omitted if the xm config was missing (or
contained an empty) 'extra='.

src/xenxs/xen_xm.c

@@ -350,7 +350,7 @@ xenParseXM(virConfPtr conf, int xendConfigVersion,
             goto cleanup;
         if (xenXMConfigCopyStringOpt(conf, "ramdisk", &def->os.initrd) < 0)
             goto cleanup;
-        if (xenXMConfigGetString(conf, "extra", &extra, "") < 0)
+        if (xenXMConfigGetString(conf, "extra", &extra, NULL) < 0)
             goto cleanup;
         if (xenXMConfigGetString(conf, "root", &root, NULL) < 0)
             goto cleanup;