block: Convert bdrv_pwrite_compressed() to BdrvChild

Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
CC: Jeff Cody <jcody@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: John Snow <jsnow@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -1868,9 +1868,10 @@ int bdrv_is_allocated_above(BlockDriverState *top,
     return 0;
 }
 
-int bdrv_pwrite_compressed(BlockDriverState *bs, int64_t offset,
+int bdrv_pwrite_compressed(BdrvChild *child, int64_t offset,
                            const void *buf, int bytes)
 {
+    BlockDriverState *bs = child->bs;
     BlockDriver *drv = bs->drv;
     int ret;