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Separate the logic for alloc desc_state and desc_extra, which will
be called separately by subsequent patches.

Use struct vring_packed to pass desc_state, desc_extra.
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220801063902.129329-20-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>