hpet/rtc: Rework RTC IRQ replacement by HPET

hpet/rtc: Rework RTC IRQ replacement by HPET

Allow the intercept the RTC IRQ for the HPET legacy mode. Then push
routing to IRQ8 completely into the HPET. This allows to turn
hpet_in_legacy_mode() into a private function. Furthermore, this stops
the RTC from clearing IRQ8 even if the HPET is in control.

This patch comes with a side effect: The RTC timers will no longer be
stoppend when there is no IRQ consumer, possibly causing a minor
performance degration. But as the guest may want to redirect the RTC to
the SCI in that mode, it should normally disable unused IRQ source
anyway.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>