vfio/pci: Virtualize zero INTx PIN if no pdev->irq

Typically pdev->irq is consistent with whether the device itself
supports INTx, where device support is reported via the PIN register.
Therefore the PIN register is often already zero if pdev->irq is zero.

Recently virtualization of the PIN register was expanded to include
the case where the device supports INTx but the platform does not
route the interrupt.  This is reported by a value of IRQ_NOTCONNECTED
on some architectures.  Other architectures just report zero for
pdev->irq.

We already disallow INTx setup if pdev->irq is zero, therefore add
this to the PIN register virtualization criteria so that a consistent
view is provided to userspace through virtualized config space and
ioctls.

Reported-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Link: https://lore.kernel.org/all/174231895238.2295.12586708771396482526.stgit@linux.ibm.com/
Tested-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Link: https://lore.kernel.org/r/20250320194145.2816379-1-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: awilliam

drivers/vfio/pci/vfio_pci_config.c

@@ -1815,7 +1815,7 @@ int vfio_config_init(struct vfio_pci_core_device *vdev)
 	}
 
 	if (!IS_ENABLED(CONFIG_VFIO_PCI_INTX) || vdev->nointx ||
-	    vdev->pdev->irq == IRQ_NOTCONNECTED)
+	    !vdev->pdev->irq || vdev->pdev->irq == IRQ_NOTCONNECTED)
 		vconfig[PCI_INTERRUPT_PIN] = 0;
 
 	ret = vfio_cap_init(vdev);