virtio_ring: split: extract the logic of attach vring

Separate the logic of attach vring, subsequent patches will call it
separately.

virtqueue_vring_init_split() completes the initialization of other
variables of vring split. We can directly use
vq->split = *vring_split to complete attach.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220801063902.129329-14-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>