devices: Add fw_cfg device

Here we add the fw_cfg device as a legacy device to the device manager.
It is guarded behind a fw_cfg flag in vmm at creation of the
DeviceManager. In this cl we implement the fw_cfg device with one
function (signature).

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

vmm/src/vm.rs

@@ -720,6 +720,13 @@ impl Vm {
             vm.sev_snp_init().map_err(Error::InitializeSevSnpVm)?;
         }
 
+        #[cfg(feature = "fw_cfg")]
+        device_manager
+            .lock()
+            .unwrap()
+            .create_fw_cfg_device()
+            .map_err(Error::DeviceManager)?;
+
         #[cfg(feature = "tdx")]
         let kernel = config
             .lock()