list volumes: also return backup type for backups

list volumes: also return backup type for backups

Otherwise, there is no storage-agnostic way to filter by backup group.

Call it subtype, to not confuse it with content type, and to be able
to re-use it for other content types than backup, if the need ever
arises.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>