vhost: remove copy threshold for async path

vhost: remove copy threshold for async path

Copy threshold has been introduced in async vhost data
path to select the appropriate copy engine to do copies
for higher efficiency.

However, it may cause packets ordering issues and also
introduces performance unpredictability.

Therefore, this patch removes copy threshold support in
async vhost data path.

Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Signed-off-by: Cheng Jiang <cheng1.jiang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>