Handle possible classtype values in eieio-persistent-read

* lisp/emacs-lisp/eieio-base.el (eieio-persistent-validate/fix-slot-value):
  The function `eieio-persistent-slot-type-is-class-p' could return
  either a single class, or a list of classes.

lisp/emacs-lisp/eieio-base.el

@@ -349,7 +349,7 @@ Second, any text properties will be stripped from strings."
                        (seq-some
                         (lambda (elt)
                           (child-of-class-p (car proposed-value) elt))
-                        classtype))
+                        (if (listp classtype) classtype (list classtype))))
 		  (eieio-persistent-convert-list-to-object
 		   proposed-value))
 		 (t