agent/sandbox:  Don't update cpuset when ncpus = 0
When receiving an OnlineCpuMemory RPC, if the number of CPUs to be
made available is 0, then updating the cpusets is a redundant operation.
Fixes:
@@ -233,6 +233,10 @@ impl Sandbox {
online_memory(&self.logger)?;
}
+ if req.nb_cpus == 0 {
+ return Ok(());
+ }
+
let guest_cpuset = rustjail_cgroups::fs::get_guest_cpuset()?;
for (_, ctr) in self.containers.iter() {
