vfio/pci: Clear MSI-X IRQ index always

When doing device assignment of a physical device, MSI-X can be
enabled with no vectors enabled and this sets the IRQ index to
VFIO_PCI_MSIX_IRQ_INDEX. However, when MSI-X is disabled, the IRQ
index is left untouched if no vectors are in use. Then, when INTx
is enabled, the IRQ index value is considered incompatible (set to
MSI-X) and VFIO_DEVICE_SET_IRQS fails. QEMU complains with :

qemu-system-x86_64: vfio 0000:08:00.0: Failed to set up TRIGGER eventfd signaling for interrupt INTX-0: VFIO_DEVICE_SET_IRQS failure: Invalid argument

To avoid that, unconditionaly clear the IRQ index when MSI-X is
disabled.

Buglink: https://issues.redhat.com/browse/RHEL-21293
Fixes: 5ebffa4 ("vfio/pci: use an invalid fd to enable MSI-X")
Cc: Jing Liu <jing2.liu@intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
(cherry picked from commit d2b668f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/vfio/pci.c

@@ -824,9 +824,11 @@ static void vfio_msix_disable(VFIOPCIDevice *vdev)
         }
     }
 
-    if (vdev->nr_vectors) {
-        vfio_disable_irqindex(&vdev->vbasedev, VFIO_PCI_MSIX_IRQ_INDEX);
-    }
+    /*
+     * Always clear MSI-X IRQ index. A PF device could have enabled
+     * MSI-X with no vectors. See vfio_msix_enable().
+     */
+    vfio_disable_irqindex(&vdev->vbasedev, VFIO_PCI_MSIX_IRQ_INDEX);
 
     vfio_msi_disable_common(vdev);
     vfio_intx_enable(vdev, &err);