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

Cargo.toml

@@ -51,6 +51,7 @@ members = [
     "crates/bins/input-qemu",
     "crates/bins/storage-qemu",
     "crates/drivers/input/uart",
+    "crates/drivers/input/virtio",
     "crates/hw-virtio",
 ]
 resolver = "2"
@@ -108,6 +109,7 @@ driver-framebuffer-virtio = { path = "crates/drivers/framebuffer/virtio" }
 driver-framebuffer-mock = { path = "crates/drivers/framebuffer/mock" }
 driver-block-virtio = { path = "crates/drivers/block/virtio" }
 driver-input-uart = { path = "crates/drivers/input/uart" }
+driver-input-virtio = { path = "crates/drivers/input/virtio" }
 
 # apps
 snake = { path = "crates/apps/snake" }