qcow2: Inform block layer about discard boundaries

At the qcow2 layer, discard is only possible on a per-cluster
basis; at the moment, qcow2 silently rounds any unaligned
requests to this granularity.  However, an upcoming patch will
fix a regression in the block layer ignoring too much of an
unaligned discard request, by changing the block layer to
break up a discard request at alignment boundaries; for that
to work, the block layer must know about our limits.

However, we can't go one step further by changing
qcow2_discard_clusters() to assert that requests are always
aligned, since that helper function is reached on paths
outside of the block layer.

CC: qemu-stable@nongnu.org
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit ecdbead6)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

block/qcow2.c

@@ -1204,6 +1204,7 @@ static void qcow2_refresh_limits(BlockDriverState *bs, Error **errp)
         bs->bl.request_alignment = BDRV_SECTOR_SIZE;
     }
     bs->bl.pwrite_zeroes_alignment = s->cluster_size;
+    bs->bl.pdiscard_alignment = s->cluster_size;
 }
 
 static int qcow2_set_key(BlockDriverState *bs, const char *key)