virtio: allow mapping up to max queue size

It's a loop from i < num_sg  and the array is VIRTQUEUE_MAX_SIZE - so
it's OK if the value read is VIRTQUEUE_MAX_SIZE.

Not a big problem in practice as people don't use
such big queues, but it's inelegant.

Reported-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
