guest helpers: avoid checking user/token if one can abort all tasks

guest helpers: avoid checking user/token if one can abort all tasks

If the user can already stop all tasks there is no point in spending
some work on every task to check if the user could also stop if
without those powerful permissions.

To avoid to much indentation rework the filter to an early-next style.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>