vhost: fix async callback return type

vhost: fix async callback return type

The async copy device callbacks are used by async APIs to transfer data
and check completion status. Async APIs return the number of packets
successfully processed to the caller applications and no error
(negative) value is allowed for API return value. Thus, negative return
values from async device callbacks don't have meaningful usage, while
adding overhead in checking the return value validity. This patch change
the callback return values from "int" to "uint32_t" to get aligned with
async API definition.

Fixes: 78639d54563a ("vhost: introduce async enqueue registration API")

Signed-off-by: Patrick Fu <patrick.fu@intel.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>