xen/pvh: Make sure we don't use ACPI_IRQ_MODEL_PIC for SCI

xen/pvh: Make sure we don't use ACPI_IRQ_MODEL_PIC for SCI

Since we are not using PIC and (at least currently) don't have IOAPIC
we want to make sure that acpi_irq_model doesn't stay set to
ACPI_IRQ_MODEL_PIC (which is the default value). If we allowed it to
stay then acpi_os_install_interrupt_handler() would try (and fail) to
request_irq() for PIC.

Instead we set the model to ACPI_IRQ_MODEL_PLATFORM which will prevent
this from happening.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>