virtio: use LGUEST_VRING_ALIGN instead of relying on pagesize

This doesn't really matter, since lguest is i386 only at the moment,
but we could actually choose a different value.  (lguest doesn't have
a guarenteed ABI).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/lguest/lguest_device.c

@@ -250,7 +250,7 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 	/* Figure out how many pages the ring will take, and map that memory */
 	lvq->pages = lguest_map((unsigned long)lvq->config.pfn << PAGE_SHIFT,
 				DIV_ROUND_UP(vring_size(lvq->config.num,
-							PAGE_SIZE),
+							LGUEST_VRING_ALIGN),
 					     PAGE_SIZE));
 	if (!lvq->pages) {
 		err = -ENOMEM;