examples/vhost: fix ioat ring space in callbacks

examples/vhost: fix ioat ring space in callbacks

We use ioat ring space for determining if ioat callbacks can enqueue a
packet to ioat device. But there is one slot can't be used in ioat
ring due to the ioat driver design, so we need to reduce one slot in
ioat ring to prevent ring size mismatch in ioat callbacks.

Fixes: 2aa47e94bfb2 ("examples/vhost: add ioat ring space count and check")
Cc: stable@dpdk.org
Signed-off-by: Cheng Jiang <cheng1.jiang@intel.com>
Reviewed-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>