next-kbd: convert to use qemu_input_handler_register()

next-kbd: convert to use qemu_input_handler_register()

Convert the next-kbd device from the legacy UI qemu_add_kbd_event_handler()
function to use qemu_input_handler_register().

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20241106120928.242443-2-mark.cave-ayland@ilande.co.uk>
[thuth: Removed the NEXTKBD_NO_KEY definition - replaced by 0 now]
Signed-off-by: Thomas Huth <thuth@redhat.com>