guest helpers: add helper to abort active guest tasks of a certain type

guest helpers: add helper to abort active guest tasks of a certain type

Given a `(type, user, vmid)` tuple, the helper aborts all tasks of the
given `type` for guest `vmid` that `user` is allowed to abort:

- If `user` has `Sys.Modify` on the node, they can abort any task
- If `user` is an API token, it can abort any task it started itself
- If `user` is a user, they can abort any task started by themselves
  or one of their API tokens.

The helper is used to overrule any active qmshutdown/vzshutdown tasks
when attempting to stop a VM/CT (if requested).

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>