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: #1172

Signed-off-by: Maruth Goyal <maruthgoyal@gmail.com>

src/agent/src/sandbox.rs

@@ -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() {