virtio_ring: introduce virtqueue_init()

Separate the logic of virtqueue initialization. These variables should
be reset during reset.

This logic can be called independently when implementing resize/reset
later.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220801063902.129329-7-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>