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/lib.rs

@@ -405,6 +405,8 @@ pub fn feature_list() -> Vec<String> {
         "dbus_api".to_string(),
         #[cfg(feature = "dhat-heap")]
         "dhat-heap".to_string(),
+        #[cfg(feature = "fw_cfg")]
+        "fw_cfg".to_string(),
         #[cfg(feature = "guest_debug")]
         "guest_debug".to_string(),
         #[cfg(feature = "igvm")]