chardev: report a simpler error about duplicated id

Report:
  "Chardev with id 'char2' already exists"
Rather than:
  "Failed to add chardev 'char2': duplicate yank instance"

Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>

Author: elmarco

chardev/char.c

@@ -1036,6 +1036,11 @@ ChardevReturn *qmp_chardev_add(const char *id, ChardevBackend *backend,
     ChardevReturn *ret;
     g_autoptr(Chardev) chr = NULL;
 
+    if (qemu_chr_find(id)) {
+        error_setg(errp, "Chardev with id '%s' already exists", id);
+        return NULL;
+    }
+
     cc = char_get_class(ChardevBackendKind_str(backend->type), errp);
     if (!cc) {
         goto err;