virtio-net: fix the upper bound when trying to delete queues

Virtqueue were indexed from zero, so don't delete virtqueue whose
index is n->max_queues * 2 + 1.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>