vmm, devices: Add firmware debug port device

OVMF and other standard firmwares use I/O port 0x402 as a simple debug
port by writing ASCII characters to it. This is gated under a feature
that is not enabled by default.

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

devices/src/legacy/mod.rs

@@ -7,10 +7,14 @@
 
 #[cfg(feature = "cmos")]
 mod cmos;
+#[cfg(feature = "fwdebug")]
+mod fwdebug;
 mod i8042;
 mod serial;
 
 #[cfg(feature = "cmos")]
 pub use self::cmos::Cmos;
+#[cfg(feature = "fwdebug")]
+pub use self::fwdebug::FwDebugDevice;
 pub use self::i8042::I8042Device;
 pub use self::serial::Serial;