I don't think you have to look for Halal restaurants particularly. Both of Chengdu and Xian(https://www.travelchinaguide.com/cityguides/xian.htm) do have so many muslim restaurants, especially in Xi'an. Suggested that you choose the hostel close to these tourist attractions or transportation hub. The Han Tang Inn Youth Hostel is located in walking-distance to the noted Xian Moslem Street(https://www.travelchinaguide.com/attraction/shaanxi/xian/moslem-street.htm), where you could find the Great Mosque and so many halal restaurants. On the other hand, you could consider the Bell Tower Youth Hostel which is a little closer to the street and just located in the city center-the Bell Tower of Xian(https://www.travelchinaguide.com/attraction/shaanxi/xian/bell.htm).