feat: VirtIO keyboard input for QEMU GUI

- Add VirtIO input device driver (device ID 18) reading from eventq
- Translate Linux keycodes (WASD, arrows) to InputEvent
- Kernel probes for VirtIO input device, falls back to UART
- Add -device virtio-keyboard-device to run-gui Makefile target
- input-qemu tries VirtIO first, falls back to UART

crates/bins/input-qemu/src/main.rs

@@ -3,14 +3,30 @@
 
 extern crate userspace_rt;
 
-use driver_input_uart::UartInputDevice;
 use types::bounded::CapSlot;
 
 #[unsafe(no_mangle)]
 extern "C" fn main() {
-    let device = UartInputDevice::new();
-    let mut srv = input_server::server::InputServer::new(device);
     let ep = CapSlot::new(0).unwrap();
-    server_framework::serve(&mut srv, ep);
+
+    if let Some(dev) = driver_input_virtio::VirtioInputDevice::try_new() {
+        run_input(dev, ep);
+    } else {
+        run_uart(ep);
+    }
+
     loop { userspace::syscall::yield_now(); }
 }
+
+#[inline(never)]
+fn run_input(device: driver_input_virtio::VirtioInputDevice, ep: CapSlot) {
+    let mut srv = input_server::server::InputServer::new(device);
+    server_framework::serve(&mut srv, ep);
+}
+
+#[inline(never)]
+fn run_uart(ep: CapSlot) {
+    let device = driver_input_uart::UartInputDevice::new();
+    let mut srv = input_server::server::InputServer::new(device);
+    server_framework::serve(&mut srv, ep);
+}