qmeventd: explicitly close() pidfds

qmeventd: explicitly close() pidfds

In most circumstances a pidfd gets closed automatically once the child
dies, and that *should* be guaranteed by us calling SIGKILL - however,
it seems that sometimes that doesn't happen, leading to leaked file
descriptors[0].

Also add a small note to verbose mode showing when the late-cleanup
actually happens, helped during debug.

[0] https://forum.proxmox.com/threads/cannot-shutdown-vm.83911/

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>