virtio-pci: error out when both legacy and modern modes are disabled

Without presuming if we got there because of a user mistake or some
more subtle bug in the tooling, it really does not make sense to
implement a non-functional device.
Signed-off-by: NGreg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: NMarcel Apfelbaum <marcel@redhat.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-pci.c

@@ -1776,6 +1776,14 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error **errp)
         proxy->disable_legacy = pcie_port ? ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
     }
 
+    if (!(virtio_pci_modern(proxy) || virtio_pci_legacy(proxy))) {
+        error_setg(errp, "device cannot work as neither modern nor legacy mode"
+                   " is enabled");
+        error_append_hint(errp, "Set either disable-modern or disable-legacy"
+                          " to off\n");
+        return;
+    }
+
     if (pcie_port && pci_is_express(pci_dev)) {
         int pos;