vmm: Include device tree in vm.info API

The DeviceNode cannot be fully represented as it embeds a Rust style
enum (i.e. with data) which is instead represented by a simple
associative array.

Fixes: #1167

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

vmm/src/device_manager.rs

@@ -3134,6 +3134,10 @@ impl DeviceManager {
 
         0
     }
+
+    pub fn device_tree(&self) -> Arc<Mutex<DeviceTree>> {
+        self.device_tree.clone()
+    }
 }
 
 #[cfg(feature = "acpi")]