Use BootTimer to measure hotplug latency

Change the boot timer timestamp to be public, so that the timer can also
be used for measuring hotplugging

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>

Author: JamesC1305

src/vmm/src/devices/pseudo/boot_timer.rs

@@ -10,7 +10,7 @@ const MAGIC_VALUE_SIGNAL_GUEST_BOOT_COMPLETE: u8 = 123;
 /// Pseudo device to record the kernel boot time.
 #[derive(Debug)]
 pub struct BootTimer {
-    start_ts: TimestampUs,
+    pub start_ts: TimestampUs,
 }
 
 impl BootTimer {

src/vmm/src/lib.rs

@@ -689,6 +689,14 @@ impl Vmm {
 
         self.acpi_device_manager.notify_cpu_container()?;
 
+        #[cfg(test)]
+        if let Some(devices::BusDevice::BootTimer(timer)) =
+            self.get_bus_device(DeviceType::BootTimer, "BootTimer")
+        {
+            let mut locked_timer = timer.lock().expect("Poisoned lock");
+            locked_timer.start_ts = utils::time::TimestampUs::default();
+        }
+
         Ok(new_machine_config)
     }