x86/HVM: fix legacy PIC check in pt_update_irq()

x86/HVM: fix legacy PIC check in pt_update_irq()

Depending on the IRQ we need to
- not look at the PIC at all is this is the LAPIC timer (in that case
  we're dealing with a vector number rather than an IRQ one),
- not look at the PIC for any non-legacy interrupt,
- look at the correct PIC for the IRQ (which will always be PIC 2 for
  the RTC, and possibly also for HPET).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Roger Pau Monné <roger.pau@citrix.com> (FreeBSD guest)
Reviewed-by: Tim Deegan <tim@xen.org>