hpet: inverse polarity when pin above ISA_NUM_IRQS

According to hpet spec, hpet irq is high active. But according to
ICH spec, there is inversion before the input of ioapic. So the OS
will expect low active on this IRQ line. (On bare metal, if OS driver
claims high active on this line, spurious irq is generated)

We fold the emulation of this inversion inside the hpet logic.

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>