Separate the logic of packed to create vring queue.

This feature is required for subsequent virtuqueue reset vring.
Signed-off-by: NXuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Message-Id: <20220801063902.129329-19-xuanzhuo@linux.alibaba.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>