virtio_ring: unify logic of virtqueue_poll() and more_used()

virtio_ring: unify logic of virtqueue_poll() and more_used()

This patch unifies the logic of virtqueue_poll() and more_used() for
better code reusing and ease the future in order implementation.

Acked-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20251230064649.55597-4-jasowang@redhat.com>