xl: Fix adding additional config cmdline parameters

When checking the size of the buffer we hold for additional
config parameters passed on the command line we should take the
size of the to-be-added string into account. While at it, rework
the implementation to be cleaner and safer.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/xl_cmdimpl.c

@@ -3179,21 +3179,17 @@ int main_create(int argc, char **argv)
         }
     }
 
-    memset(extra_config, 0, sizeof(extra_config));
-    while (optind < argc) {
-        if ((p = strchr(argv[optind], '='))) {
-            if (strlen(extra_config) + 1 < sizeof(extra_config)) {
-                if (strlen(extra_config))
-                    strcat(extra_config, "\n");
-                strcat(extra_config, argv[optind]);
-            }
+    extra_config[0] = '\0';
+    for (p = extra_config; optind < argc; optind++) {
+        if (strchr(argv[optind], '=') != NULL) {
+            p += snprintf(p, sizeof(extra_config) - (p - extra_config),
+                "%s\n", argv[optind]);
         } else if (!filename) {
             filename = argv[optind];
         } else {
             help("create");
             return 2;
         }
-        optind++;
     }
 
     memset(&dom_info, 0, sizeof(dom_info));