[libvirt] RFC backup API

Hi all, It's been already quite a long time since qemu implemented QMP "drive-backup" command to create block devices backups. Even more, since qemu 2.4 there is a possibility to create incremental backups. Though it is possible to backup all attached to a domain disk drives by combining them into a single QMP transaction command, this way of creating them, not to mention managing, remains inconvenient for an end user of libvirt. Moreover, creating a single drive backup via QMP interface isn't handy either. That said, it looks reasonable to introduce a *new backup API* based on QMP "drive-backup" facilities. Though we can start from a single simple function, allowing to create a disk backup by means of QMP "drive-backup" command, I'd like to discuss here the level of management libvirt could provide for backup operations. To begin with, here is the preliminary list of possible functions that I think make sense for libvirt API. virDomainCreateBackup - which creates a backup full/incremental of all/selected disks, virListBackups - which lists all backups created for a particular domain/target, virRestoreBackup - which restores all/selected disks from a backup, virDeleteBackup - which deletes all/selected disks from a backup. It looks like backup management functions, except create one, shouldn't be or might not be bound to a particular domain and we could possibly leverage storage pool API with some extension. Specifically, volume definition could be extended with necessary meta data related to backups. The *question* is: if the whole idea about this new API as described above or something similar makes sense? If yes, then let's find out requirements for it (if any) and I will try to prepare a patch set with the first RFC implementation to discuss the API in more details. Looking forward for your opinions on the matter. Maxim