xen/pci: Use acpi_noirq_set() helper to avoid #ifdef

No need to keep an #ifdef since we have a helper (in x86 world).  Use it
instead.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>