xen/mce: fix up xen_late_init_mcelog() error handling

Static checkers complain about the missing call to misc_deregister() if bind_virq_for_mce() fails.

Also I reversed the tests so that we do error handling instead of success handling. That way we just have a series of function calls instead of the more complicated nested if statements in the original code. Let's preserve the error codes as well.