qdict: Make qdict_get_qlist() safe like qdict_get_qdict()

Commit 89cad9f changed qdict_get_qdict() to return NULL instead of
crash when the key doesn't exist or its value isn't a QDict.
Commit 2d6421a neglected to do the same for qdict_get_qlist().
Correct that, and update the function comments.

qdict_get_obj() is now unused, remove.

Backports commit b25f23e7dbc6bc0dcda010222a4f178669d1aedc from qemu

qemu/qobject/qdict.c

@@ -177,20 +177,6 @@ size_t qdict_size(const QDict *qdict)
     return qdict->size;
 }
 
-/**
- * qdict_get_obj(): Get a QObject of a specific type
- */
-static QObject *qdict_get_obj(const QDict *qdict, const char *key, QType type)
-{
-    QObject *obj;
-
-    obj = qdict_get(qdict, key);
-    assert(obj != NULL);
-    assert(qobject_type(obj) == type);
-
-    return obj;
-}
-
 /**
  * qdict_get_double(): Get an number mapped by 'key'
  *
@@ -241,25 +227,15 @@ bool qdict_get_bool(const QDict *qdict, const char *key)
 }
 
 /**
- * qdict_get_qlist(): Get the QList mapped by 'key'
+ * qdict_get_qlist(): If @qdict maps @key to a QList, return it, else NULL.
- *
- * This function assumes that 'key' exists and it stores a
- * QList object.
- *
- * Return QList mapped by 'key'.
  */
 QList *qdict_get_qlist(const QDict *qdict, const char *key)
 {
-    return qobject_to_qlist(qdict_get_obj(qdict, key, QTYPE_QLIST));
+    return qobject_to_qlist(qdict_get(qdict, key));
 }
 
 /**
- * qdict_get_qdict(): Get the QDict mapped by 'key'
+ * qdict_get_qdict(): If @qdict maps @key to a QDict, return it, else NULL.
- *
- * This function assumes that 'key' exists and it stores a
- * QDict object.
- *
- * Return QDict mapped by 'key'.
  */
 QDict *qdict_get_qdict(const QDict *qdict, const char *key)
 {