hpet: enable to entitle more irq pins for hpet

hpet: enable to entitle more irq pins for hpet

Owning to some different hardware design, piix and q35 need
different compat. So making them diverge.

On q35, IRQ2/8 can be reserved for hpet timer 0/1. And pin 16~23
can be assigned to hpet as guest chooses. So we introduce intcap
property to do that.

Consider the compat and piix/q35, we finally have the following
value for intcap: For piix, hpet's intcap is hard coded as IRQ2.
For pc-q35-1.7 and earlier, we use IRQ2 for compat reason. Otherwise
IRQ2, IRQ8, and IRQ16~23 are allowed.

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