Change the type of buff_idx from uint64_t to uint32_t to fix coverity
issue.
Coverity issue: 366264
Fixes: a68ba8e0a6b6 ("examples/vhost: refactor vhost data path")
Signed-off-by: Cheng Jiang <cheng1.jiang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>