x86/cpuidle: push parked CPUs into deeper sleep states when possible

x86/cpuidle: push parked CPUs into deeper sleep states when possible

When the mwait-idle driver isn't used, C-state information becomes
available only in the course of Dom0 starting up. Use the provided data
to allow parked CPUs to sleep in a more energy efficient way, by waking
them briefly (via NMI) once the data has been recorded.

This involves re-arranging how/when the governor's ->enable() hook gets
invoked. The changes there include addition of so far missing error
handling in the respective CPU notifier handlers.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>