fix #5067: vzdump: include total backup size in notification

fix #5067: vzdump: include total backup size in notification

The old backup job notification mails from before the notification
system overhaul included the total time as well as the total size.

The total size was missing from the new, template-based backup report,
thus we add it back in this commit.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
[FE: style fix - use parentheses for returning list]
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>