hw/input/stellaris_gamepad: Convert to qemu_input_handler_register()

hw/input/stellaris_gamepad: Convert to qemu_input_handler_register()

Now that we have converted to qdev, we can use the newer
qemu_input_handler_register() API rather than the legacy
qemu_add_kbd_event_handler().

Since we only have one user, take the opportunity to convert
from scancodes to QCodes, rather than using
qemu_input_key_value_to_scancode() (which adds an 0xe0
prefix and encodes up/down indication in the scancode,
which our old handler function then had to reverse). That
lets us drop the old state field which was tracking whether
we were halfway through a two-byte scancode.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20231030114802.3671871-7-peter.maydell@linaro.org