hw/xen: Support GSI mapping to PIRQ

hw/xen: Support GSI mapping to PIRQ

If I advertise XENFEAT_hvm_pirqs then a guest now boots successfully as
long as I tell it 'pci=nomsi'.

[root@localhost ~]# cat /proc/interrupts
           CPU0
  0:         52   IO-APIC   2-edge      timer
  1:         16  xen-pirq   1-ioapic-edge  i8042
  4:       1534  xen-pirq   4-ioapic-edge  ttyS0
  8:          1  xen-pirq   8-ioapic-edge  rtc0
  9:          0  xen-pirq   9-ioapic-level  acpi
 11:       5648  xen-pirq  11-ioapic-level  ahci[0000:00:04.0]
 12:        257  xen-pirq  12-ioapic-edge  i8042
...

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>