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>

src/main.rs

@@ -853,6 +853,8 @@ fn main() {
     compile_error!("Feature 'tdx' and 'sev_snp' are mutually exclusive.");
     #[cfg(all(feature = "sev_snp", not(target_arch = "x86_64")))]
     compile_error!("Feature 'sev_snp' needs target 'x86_64'");
+    #[cfg(all(feature = "fw_cfg", target_arch = "riscv64"))]
+    compile_error!("Feature 'fw_cfg' needs targets 'x86_64' or 'aarch64'");
 
     #[cfg(feature = "dhat-heap")]
     let _profiler = dhat::Profiler::new_heap();