Increase default task limit for libvirtd

Increase default task limit for libvirtd

libvirtd can spawn threads/tasks when creating new domains for
some hypervisors such as Xen's libxl driver, quickly reaching
the cgroups pids controller default TasksMax setting of 512. When
the limit is reached, attempting to create additional domains
results in an error from the cgroups pids controller, e.g.

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

Depending on domain type and configuration, anywhere from 4-7
threads/tasks may be created by libxl when starting a domain.
In order to support 4096 domains, similar to commit 27cd763500,
increase the TasksMax setting in libvirtd.service to
4096 * 8 = 32768 tasks.