block: Convert .bdrv_aio_discard() to byte-based

Another step towards byte-based interfaces everywhere.  Replace
the sector-based driver callback .bdrv_aio_discard() with a new
byte-based .bdrv_aio_pdiscard().  Only raw-posix and RBD drivers
are affected, so it was not worth splitting into multiple patches.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1468624988-423-9-git-send-email-eblake@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/block/block_int.h

@@ -142,8 +142,8 @@ struct BlockDriver {
         BlockCompletionFunc *cb, void *opaque);
     BlockAIOCB *(*bdrv_aio_flush)(BlockDriverState *bs,
         BlockCompletionFunc *cb, void *opaque);
-    BlockAIOCB *(*bdrv_aio_discard)(BlockDriverState *bs,
-        int64_t sector_num, int nb_sectors,
+    BlockAIOCB *(*bdrv_aio_pdiscard)(BlockDriverState *bs,
+        int64_t offset, int count,
         BlockCompletionFunc *cb, void *opaque);
 
     int coroutine_fn (*bdrv_co_readv)(BlockDriverState *bs,