xen/pt: allow passthrough of devices with bogus interrupt pin

xen/pt: allow passthrough of devices with bogus interrupt pin

For some pci device, even its PCI_INTERRUPT_PIN is not 0, it actually
doesn't support INTx mode, so its machine irq read from host sysfs is 0.
In that case, report PCI_INTERRUPT_PIN as 0 to guest and let passthrough
continue.

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Zhao Yan <yan.y.zhao@intel.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>