virtio-mmio: read/write the hi 32 features for mmio

Under mmio, when we read the feature from the device, we should read the
high 32-bit part. Similarly, when writing the feature back, we should
also write back the high 32-bit feature.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20221114035818.109511-2-xuanzhuo@linux.alibaba.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>