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/Cargo.toml

@@ -15,3 +15,4 @@ userspace = { workspace = true }
 server-framework = { workspace = true }
 input-server = { workspace = true }
 driver-input-uart = { workspace = true }
+driver-input-virtio = { workspace = true }