Avoid IRQs occupied by ISA IRQs when allocating IRQs for PCI link devices,
otherwise it may cause interrupt storm due to incompatible pin attributes.

This issue was triggered on a KVM virtual machine, which
 1) uses IRQ9 for SCI in high level mode.
 2) defines an PCI interrupt link device (LNKS) with IRQ9 as the only
    possible irq.
 3) has an PCI device referring to link device LNKS.
So it causes interrupt storm when enabling the PCI device because PCI IRQ
works in low level mode.
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>