Revert "qobject: let object_property_get_str() use new API"

Commit aafb21a "qobject: let object_property_get_str() use new API"
isn't much of a simplification.  Not worth having
object_property_get_str() differ from the other
object_property_get_FOO().  Revert.

This reverts commit aafb21a.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201211171152.146877-12-armbru@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>

qom/object.c

@@ -1415,15 +1415,18 @@ char *object_property_get_str(Object *obj, const char *name,
                               Error **errp)
 {
     QObject *ret = object_property_get_qobject(obj, name, errp);
+    QString *qstring;
     char *retval;
 
     if (!ret) {
         return NULL;
     }
-
-    retval = g_strdup(qobject_get_try_str(ret));
-    if (!retval) {
+    qstring = qobject_to(QString, ret);
+    if (!qstring) {
         error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name, "string");
+        retval = NULL;
+    } else {
+        retval = g_strdup(qstring_get_str(qstring));
     }
 
     qobject_unref(ret);