vdpa_sim: Use iova_shift() for the size passed to alloc_iova()

The size passed to alloc_iova() should be the size of page frames.
Now we use byte granularity for the iova domain, so it's safe to
pass the size in bytes to alloc_iova(). But it would be better to use
iova_shift() for the size to avoid future bugs if we change granularity.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20210809100923.38-1-xieyongji@bytedance.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>