vdpa: Check for existence of opts.vhostdev

Since net_init_vhost_vdpa is trying to open it. Not specifying it in the
command line crash qemu.

Fixes: 7327813d17 ("vhost-vdpa: open device fd in net_init_vhost_vdpa()")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20211112193431.2379298-3-eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

net/vhost-vdpa.c

@@ -260,6 +260,10 @@ int net_init_vhost_vdpa(const Netdev *netdev, const char *name,
 
     assert(netdev->type == NET_CLIENT_DRIVER_VHOST_VDPA);
     opts = &netdev->u.vhost_vdpa;
+    if (!opts->vhostdev) {
+        error_setg(errp, "vdpa character device not specified with vhostdev");
+        return -1;
+    }
 
     vdpa_device_fd = qemu_open(opts->vhostdev, O_RDWR, errp);
     if (vdpa_device_fd == -1) {