agent/device: update_spec_device_list() should error if dev not found

If update_spec_device_list() is given a device that can't be found in the
OCI spec, it currently does nothing, and returns Ok(()).  That doesn't
seem like what we'd expect and is not what the Go agent in Kata 1 does.

Change it to return an error in that case, like Kata 1.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

src/agent/src/device.rs

@@ -262,10 +262,14 @@ fn update_spec_device_list(device: &Device, spec: &mut Spec) -> Result<()> {
                     }
                 }
             }
+            return Ok(());
         }
     }
 
-    Ok(())
+    Err(anyhow!(
+        "Should have found a matching device {} in the spec",
+        device.vm_path
+    ))
 }
 
 // device.Id should be the predicted device name (vda, vdb, ...)