block: Accept any target node for transactional blockdev-backup

Commit 0d978913 changed blockdev-backup to accept arbitrary node names
instead of device names (i.e. root nodes) for the backup target.
However, it forgot to make the same change in transactions and to update
the documentation. This patch fixes these omissions.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>

blockdev.c

@@ -1937,7 +1937,8 @@ static void blockdev_backup_prepare(BlkActionState *common, Error **errp)
 {
     BlockdevBackupState *state = DO_UPCAST(BlockdevBackupState, common, common);
     BlockdevBackup *backup;
-    BlockBackend *blk, *target;
+    BlockBackend *blk;
+    BlockDriverState *target;
     Error *local_err = NULL;
 
     assert(common->action->type == TRANSACTION_ACTION_KIND_BLOCKDEV_BACKUP);
@@ -1954,15 +1955,14 @@ static void blockdev_backup_prepare(BlkActionState *common, Error **errp)
         return;
     }
 
-    target = blk_by_name(backup->target);
+    target = bdrv_lookup_bs(backup->target, backup->target, errp);
     if (!target) {
-        error_setg(errp, "Device '%s' not found", backup->target);
         return;
     }
 
     /* AioContext is released in .clean() */
     state->aio_context = blk_get_aio_context(blk);
-    if (state->aio_context != blk_get_aio_context(target)) {
+    if (state->aio_context != bdrv_get_aio_context(target)) {
         state->aio_context = NULL;
         error_setg(errp, "Backup between two IO threads is not implemented");
         return;

qapi/block-core.json

@@ -927,7 +927,7 @@
 #
 # @device: the name of the device which should be copied.
 #
-# @target: the name of the backup target device.
+# @target: the device name or node-name of the backup target node.
 #
 # @sync: what parts of the disk image should be copied to the destination
 #        (all the disk, only the sectors allocated in the topmost image, or