vmm: Ensure default deserialisation for "amx" feature bit

This allows a migration from a binary not compiled with struct member to
be completed.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/config.rs

@@ -473,6 +473,7 @@ pub struct CpuAffinity {
 #[derive(Clone, Debug, Default, PartialEq, Eq, Deserialize, Serialize)]
 pub struct CpuFeatures {
     #[cfg(all(feature = "amx", target_arch = "x86_64"))]
+    #[serde(default)]
     pub amx: bool,
 }