blockdev: enable non-root nodes for backup source

This is needed to implement the image-fleecing workflow where we
create a temporary node backed by an active node, then start
backupdev-backup sync=none from the active node to the temp node.

In this case, the active node is now a root node AND a backing node,
so it no longer qualifies as a root node, so we loosen the restriction
on which nodes can be considered as the source for a backup.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Message-Id: <20180702194630.9360-2-jsnow@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

blockdev.c

@@ -1859,7 +1859,7 @@ static void blockdev_backup_prepare(BlkActionState *common, Error **errp)
     assert(common->action->type == TRANSACTION_ACTION_KIND_BLOCKDEV_BACKUP);
     backup = common->action->u.blockdev_backup.data;
 
-    bs = qmp_get_root_bs(backup->device, errp);
+    bs = bdrv_lookup_bs(backup->device, backup->device, errp);
     if (!bs) {
         return;
     }
@@ -3517,7 +3517,7 @@ BlockJob *do_blockdev_backup(BlockdevBackup *backup, JobTxn *txn,
         backup->compress = false;
     }
 
-    bs = qmp_get_root_bs(backup->device, errp);
+    bs = bdrv_lookup_bs(backup->device, backup->device, errp);
     if (!bs) {
         return NULL;
     }