vhost: readd assert statement

It's clear from the surrounding code that
start < end so it's enough to assert end < log_size.
However, it's better to make this explicit in case
we refactor the code again.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/vhost.c

@@ -35,6 +35,7 @@ static void vhost_dev_sync_region(struct vhost_dev *dev,
         return;
     }
     assert(end / VHOST_LOG_CHUNK < dev->log_size);
+    assert(start / VHOST_LOG_CHUNK < dev->log_size);
 
     for (;from < to; ++from) {
         vhost_log_chunk_t log;