s390x/pci: bypass vfio DMA counting when using cdev

The current code assumes that there is always a vfio group, but
that's no longer guaranteed with the iommufd backend when using
cdev.  In this case, we don't need to track the vfio dma limit
anyway.

Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Message-ID: <20231110175108.465851-2-mjrosato@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/s390x/s390-pci-vfio.c

@@ -66,6 +66,10 @@ S390PCIDMACount *s390_pci_start_dma_count(S390pciState *s,
 
     assert(vpdev);
 
+    if (!vpdev->vbasedev.group) {
+        return NULL;
+    }
+
     id = vpdev->vbasedev.group->container->fd;
 
     if (!s390_pci_update_dma_avail(id, &avail)) {