unicorn engine warning appears even when nobody is trying to use it

[rhelmot]

[15:14:51] [simuvex.plugins.unicorn] | ERROR: You are attempting to use unicorn engine support even though it or the simuvex compatibility layer isn't installed

This is because this unicorn SimEngine runs the cooldown decrement function in the _check_failed callback, regardless of the reason for failure. This should be moved to being called directly from the check function, ONLY when the state option is enabled, perhaps guarded by a few more checks.

Furthermore, we should use this as an opportunity to re-evaluate the need for a callback function at all. IMO it's a bit of a wart.