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

This function completes the variable initialization of packed vring. It
together with the logic of atatch constitutes the initialization of
vring.
Signed-off-by: NXuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Message-Id: <20220801063902.129329-21-xuanzhuo@linux.alibaba.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>