vmm: Prepare for static CPU template deprecation

Add an implementation of conversion from `&CpuTemplateType` to
`Option<StaticCpuTemplate>`, which will be used in a successive commit
to deprecate the `cpu_template` field in the `/machine-config` API.

Signed-off-by: Takahiro Itazuri <itazur@amazon.com>

src/vmm/src/cpu_config/templates.rs

@@ -63,6 +63,8 @@ pub enum CpuTemplateType {
     Static(StaticCpuTemplate),
 }
 
+// This conversion is only used for snapshot, but the static CPU template
+// information has not been saved into snapshot since v1.1.
 impl From<&Option<CpuTemplateType>> for StaticCpuTemplate {
     fn from(value: &Option<CpuTemplateType>) -> Self {
         match value {
@@ -72,6 +74,18 @@ impl From<&Option<CpuTemplateType>> for StaticCpuTemplate {
     }
 }
 
+// This conversion will be used when converting `&VmConfig` to `MachineConfig`
+// to respond `GET /machine-config` and `GET /vm`.
+#[allow(dead_code)]
+impl From<&CpuTemplateType> for StaticCpuTemplate {
+    fn from(value: &CpuTemplateType) -> Self {
+        match value {
+            CpuTemplateType::Static(template) => *template,
+            CpuTemplateType::Custom(_) => StaticCpuTemplate::None,
+        }
+    }
+}
+
 impl<'a> TryFrom<&'a [u8]> for CustomCpuTemplate {
     type Error = serde_json::Error;