virtio-balloon: Use virtio_find_vqs() helper

Use the helper virtio_find_vqs().

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210723054259.2779-1-xianting.tian@linux.alibaba.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>