proc fs tools: handle proc/stat without guest values

proc fs tools: handle proc/stat without guest values

PMG is often run as a container, and in certain environments (like
Virtuozzo 7), the last two values (guest and guest_nice) might not be
present. This led to a division by zero, because the total value was
never updated.

Reported in the community forum:
https://forum.proxmox.com/threads/106896/

Acked-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>