virtio-serial: pci: Allow MSI to be disabled

Michael noted we don't allow disabling of MSI for the virtio-serial-pci
device. Fix that.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-pci.c

@@ -500,8 +500,8 @@ static int virtio_serial_init_pci(PCIDevice *pci_dev)
     if (!vdev) {
         return -1;
     }
-    vdev->nvectors = proxy->nvectors ? proxy->nvectors
-                                     : proxy->max_virtserial_ports + 1;
+    vdev->nvectors = proxy->nvectors == -1 ? proxy->max_virtserial_ports + 1
+                                           : proxy->nvectors;
     virtio_init_pci(proxy, vdev,
                     PCI_VENDOR_ID_REDHAT_QUMRANET,
                     PCI_DEVICE_ID_VIRTIO_CONSOLE,
@@ -585,7 +585,7 @@ static PCIDeviceInfo virtio_info[] = {
         .init      = virtio_serial_init_pci,
         .exit      = virtio_exit_pci,
         .qdev.props = (Property[]) {
-            DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors, 0),
+            DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors, -1),
             DEFINE_PROP_HEX32("class", VirtIOPCIProxy, class_code, 0),
             DEFINE_VIRTIO_COMMON_FEATURES(VirtIOPCIProxy, host_features),
             DEFINE_PROP_UINT32("max_ports", VirtIOPCIProxy, max_virtserial_ports,