performance-metrics: Add vm_type to PerformanceTestOverrides

Add an optional vm_type field to PerformanceTestOverrides to
allow overriding the VM type at runtime. Include vm_type in
the Display output for override logging.

Signed-off-by: Muminul Islam <muislam@microsoft.com>

test_infra/src/lib.rs

@@ -2487,8 +2487,9 @@ pub fn extract_bar_address(output: &str, device_desc: &str, bar_index: usize) ->
     None
 }
 
-#[derive(PartialEq, Clone, Copy)]
+#[derive(PartialEq, Clone, Copy, Default)]
 pub enum GuestVmType {
+    #[default]
     Regular,
     Confidential,
 }