virtio_ring: split vring_virtqueue

Separate the two inline structures(split and packed) from the structure
vring_virtqueue.

In this way, we can use these two structures later to pass parameters
and retain temporary variables.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220801063902.129329-6-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>