vzdump: fix notifications for backing up VMs with 2+ disks to PBS

vzdump: fix notifications for backing up VMs with 2+ disks to PBS

In some situations, such as backing up VMs with 2 or more disks to
PBS, we get passed the backup archive size as a string instead of
as an integer. This led to errors rendering the notification template
down the line.

This commit explicitly casts the data from the task table to an int.
It would be a good idea to actually hunt down the places that produced
the string instead of an integer, but as a quick fix and as a
safeguard against similar lurking errors this approach is fine, IMO.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>