virtio-pci: build for uninitialized return value in vq_vector_unmask

Fixes the following:

/home/mdroth/w/qemu2.git/hw/virtio-pci.c: In function
‘kvm_virtio_pci_vector_unmask’:
/home/mdroth/w/qemu2.git/hw/virtio-pci.c:673:12: error: ‘ret’ may be
used uninitialized in this function [-Werror=uninitialized]
cc1: all warnings being treated as errors
make: *** [hw/virtio-pci.o] Error 1
make: *** Waiting for unfinished jobs....
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-pci.c

@@ -616,7 +616,7 @@ static int kvm_virtio_pci_vq_vector_unmask(VirtIOPCIProxy *proxy,
     VirtQueue *vq = virtio_get_queue(proxy->vdev, queue_no);
     EventNotifier *n = virtio_queue_get_guest_notifier(vq);
     VirtIOIRQFD *irqfd = &proxy->vector_irqfd[vector];
-    int ret;
+    int ret = 0;
 
     if (irqfd->msg.data != msg.data || irqfd->msg.address != msg.address) {
         ret = kvm_irqchip_update_msi_route(kvm_state, irqfd->virq, msg);