[libvirt] increase daemon task limit

[Jim Fehlig]

I recently received a report of a libvirt+Xen installation reaching the pids 
cgroup controller TasksMax limit

kernel: [71282.213347] cgroup: fork rejected by pids controller in 
/system.slice/libvirtd.service

The default setting of TaskMax is 512 on this system

# cat /sys/fs/cgroup/pids/system.slice/libvirtd.service/pids.max
512

Depending on domain type and configuration, I've noticed between 5-7 tasks (IO 
threads, qemu process, etc.) are created when starting a domain. It doesn't take 
too many domains before the 512 limit is reached.

LimitNOFILE was recently changed by commit 27cd763500 to support 4096 domains. 
Following similar logic, would it be ok to increase TasksMax to 32768? That 
would accommodate 4096 domains with 8 tasks each. TasksMax also supports the 
special value of "infinity", but that seems a bit aggressive to me.

Regards,
Jim