vhost: simplify async copy completion

vhost: simplify async copy completion

Current async ops allows check_completed_copies() callback to return
arbitrary number of async iov segments finished from backend async
devices. This design creates complexity for vhost to handle breaking
transfer of a single packet (i.e. transfer completes in the middle
of a async descriptor) and prevents application callbacks from
leveraging hardware capability to offload the work. Thus, this patch
enforces the check_completed_copies() callback to return the number
of async memory descriptors, which is aligned with async transfer
data ops callbacks. vhost async data path are revised to work with
new ops define, which provides a clean and simplified processing.

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