fix(xapi): add missing file

Introduced by b12c17947

Thanks @Danp2.

@xen-orchestra/xapi/_AggregateError.js

@@ -0,0 +1,9 @@
+'use strict'
+
+// TODO: remove when Node >=15.0
+module.exports = class AggregateError extends Error {
+  constructor(errors, message) {
+    super(message)
+    this.errors = errors
+  }
+}