devices: Implement DMA for fw_cfg device

We pass a reference to the guest memory when we create the device
in DeviceManager. This allows us to access the guest memory for DMA.

Signed-off-by: Alex Orozco <alexorozco@google.com>

vmm/src/device_manager.rs

@@ -1477,7 +1477,9 @@ impl DeviceManager {
 
     #[cfg(feature = "fw_cfg")]
     pub fn create_fw_cfg_device(&mut self) -> Result<(), DeviceManagerError> {
-        let fw_cfg = Arc::new(Mutex::new(devices::legacy::FwCfg::new()));
+        let fw_cfg = Arc::new(Mutex::new(devices::legacy::FwCfg::new(
+            self.memory_manager.lock().as_ref().unwrap().guest_memory(),
+        )));
 
         self.fw_cfg = Some(fw_cfg.clone());