vhost: fix async callbacks return type

vhost: fix async callbacks return type

The async vhost callback ops should return negative value when there
are something wrong in the callback, so the return type should be
changed into int32_t. The issue in vhost example is also fixed.

Fixes: cd6760da1076 ("vhost: introduce async enqueue for split ring")
Fixes: 819a71685826 ("vhost: fix async callback return type")
Fixes: 6b3c81db8bb7 ("vhost: simplify async copy completion")
Fixes: abec60e7115d ("examples/vhost: support vhost async data path")
Fixes: 6e9a9d2a02ae ("examples/vhost: fix ioat dependency")
Fixes: 873e8dad6f49 ("vhost: support packed ring in async datapath")
Cc: stable@dpdk.org
Signed-off-by: Cheng Jiang <cheng1.jiang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>