Use the new array size when calling realloc(). Note that this queue
stores events read from a host evdev device, i.e., it is not
guest-controlled.

While here, use the cdefs.h __unused macro and use them correctly, i.e.,
don't mark "arg" as unused when it isn't.

Fixes: 054accac71e0 ("Add a virtio-input device emulation.")
Reported by: Brian Delgado from Intel using Anthropic Opus 4.8