Add reset-state function for backups

Since there are volume reset-state function and snapshot reset-state function, backup also needs reset-state as well.
Use backup-create routine as an example to illustrate what benefit we can get from backup-reset function.
Backup-create routine first backup volume and metadata, and then update the status of volume and backup. If database just went down after update the volume's status to 'available', leaving the backup's status to be 'creating' without having methods to deal with through API.
If we have reset-state API, we could change the backup's status to available without manually change the backup's status in db, which could be a good mean for maintainers with little risk.