qapi: Make more of qobject_to()

This patch reworks some places which use either qobject_type() checks
plus qobject_to(), where the latter alone is sufficient, or NULL checks
plus qobject_type() checks where we can simply do a qobject_to() != NULL
check.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20180224154033.29559-6-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: rebase to qobject_to() parameter ordering]
Signed-off-by: Eric Blake <eblake@redhat.com>
diff --git a/qobject/json-parser.c b/qobject/json-parser.c
index 055c7f0..769b960 100644
--- a/qobject/json-parser.c
+++ b/qobject/json-parser.c
@@ -276,7 +276,8 @@
  */
 static int parse_pair(JSONParserContext *ctxt, QDict *dict, va_list *ap)
 {
-    QObject *key = NULL, *value;
+    QObject *value;
+    QString *key = NULL;
     JSONToken *peek, *token;
 
     peek = parser_context_peek_token(ctxt);
@@ -285,8 +286,8 @@
         goto out;
     }
 
-    key = parse_value(ctxt, ap);
-    if (!key || qobject_type(key) != QTYPE_QSTRING) {
+    key = qobject_to(QString, parse_value(ctxt, ap));
+    if (!key) {
         parse_error(ctxt, peek, "key is not a string in object");
         goto out;
     }
@@ -308,14 +309,14 @@
         goto out;
     }
 
-    qdict_put_obj(dict, qstring_get_str(qobject_to(QString, key)), value);
+    qdict_put_obj(dict, qstring_get_str(key), value);
 
-    qobject_decref(key);
+    QDECREF(key);
 
     return 0;
 
 out:
-    qobject_decref(key);
+    QDECREF(key);
 
     return -1;
 }