vmm: vm_config: apply serde defaults to FwCfgConfig

FwCfgConfig already has a Default impl (e820/kernel/cmdline/initramfs/
acpi_tables = true, items = None), but deserialization did not use it:
without serde default every field was mandatory, so any caller building
a payload config over the API had to spell out the whole object even to
flip a single flag.

Add a container-level #[serde(default)] so missing fields fall back to
FwCfgConfig::default(). The container form is required here because the
defaults are all true; a per-field #[serde(default)] would resolve bool
to false and contradict the Default impl.

This lets callers send only the fields that differ from the defaults.

Assisted-by: Claude:Opus-4.8
Signed-off-by: Ruben Hakobyan <hruben@meta.com>

vmm/src/vm_config.rs

@@ -939,6 +939,7 @@ pub struct PayloadConfig {
 #[cfg(feature = "fw_cfg")]
 #[serde_with::skip_serializing_none]
 #[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
+#[serde(default)]
 pub struct FwCfgConfig {
     pub e820: bool,
     pub kernel: bool,