blockdev: enable non-root nodes for transaction drive-backup source

We forget to enable it for transaction .prepare, while it is already
enabled in do_drive_backup since commit a2d665c
    "blockdev: loosen restrictions on drive-backup source node"

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190618140804.59214-1-vsementsov@virtuozzo.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

blockdev.c

@@ -1774,7 +1774,7 @@ static void drive_backup_prepare(BlkActionState *common, Error **errp)
     assert(common->action->type == TRANSACTION_ACTION_KIND_DRIVE_BACKUP);
     backup = common->action->u.drive_backup.data;
 
-    bs = qmp_get_root_bs(backup->device, errp);
+    bs = bdrv_lookup_bs(backup->device, backup->device, errp);
     if (!bs) {
         return;
     }