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: N"Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 93725140)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/virtio/virtio.c

@@ -427,7 +427,7 @@ void virtqueue_map_sg(struct iovec *sg, hwaddr *addr,
     unsigned int i;
     hwaddr len;
 
-    if (num_sg >= VIRTQUEUE_MAX_SIZE) {
+    if (num_sg > VIRTQUEUE_MAX_SIZE) {
         error_report("virtio: map attempt out of bounds: %zd > %d",
                      num_sg, VIRTQUEUE_MAX_SIZE);
         exit(1);