prune backups: make vmid filtering more robust

prune backups: make vmid filtering more robust

by relying on archive_info's vmid first. archive_info is already used to
determine if it's a standard name, and in that case the vmid is certainly set.

Also add asserts to make sure we got what we expected.

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