xen/pci: Move the allocation of IRQs when there are no IOAPIC's to the end

.. which means we can preset of NR_IRQS_LEGACY interrupts using
the 'acpi_get_override_irq' API before this loop.
This means that we can get the IRQ's polarity (and trigger) from either
the ACPI (or MP); or use the default values. This fixes a bug if we did
not have an IOAPIC we would not been able to preset the IRQ's polarity
if the MP table existed.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>