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>