virtio-pci: Check for virtio_blk_init() failure

It can't actually fail now, but the next commit will change that.

s390_virtio_blk_init() already checks for failure, but
virtio_blk_init_pci() doesn't.  Fix that.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/virtio-pci.c

@@ -551,6 +551,9 @@ static int virtio_blk_init_pci(PCIDevice *pci_dev)
         return -1;
     }
     vdev = virtio_blk_init(&pci_dev->qdev, &proxy->block);
+    if (!vdev) {
+        return -1;
+    }
     vdev->nvectors = proxy->nvectors;
     virtio_init_pci(proxy, vdev,
                     PCI_VENDOR_ID_REDHAT_QUMRANET,