src: set max open file limit to match systemd >= 240 defaults

src: set max open file limit to match systemd >= 240 defaults

Since systemd 240, all services get an open file hard limit of
500k, and a soft limit of 1024. This limit means apps are safe
to use select() by default which is limited to 1024 FDs. Apps
which don't use select() are expected to simply set their soft
limit to match the hard limit during startup.

With our current unit file settings we've been effectively
reducing the max open files we have on most modern systems.

https://gitlab.com/libvirt/libvirt/-/issues/489
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>