refactor: Make MachineConfigUpdate::is_empty forward compatible

Instead of listing out each field in the function (which is error prone
in case more fields get added, because the compiler will not be able to
detect that they might be missing from the `is_empty` condition),
compare against `Default::default`, which will automatically include all
future fields in the comparision, and `Default::default()` will always
set all fields to `None`.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

src/vmm/src/vmm_config/machine_config.rs

@@ -60,7 +60,7 @@ impl Default for MachineConfig {
 /// All fields are optional, but at least one needs to be specified.
 /// If a field is `Some(value)` then we assume an update is requested
 /// for that field.
-#[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
+#[derive(Clone, Default, Debug, PartialEq, Eq, Deserialize, Serialize)]
 #[serde(deny_unknown_fields)]
 pub struct MachineConfigUpdate {
     /// Number of vcpu to start.
@@ -85,16 +85,7 @@ impl MachineConfigUpdate {
     /// Returns `true` if all fields are set to `None` which means that there is nothing
     /// to be updated.
     pub fn is_empty(&self) -> bool {
-        if self.vcpu_count.is_none()
-            && self.mem_size_mib.is_none()
-            && self.cpu_template.is_none()
-            && self.smt.is_none()
-            && self.track_dirty_pages.is_none()
-        {
-            return true;
-        }
-
-        false
+        self == &Default::default()
     }
 }