vhost: simplify async IO vectors

vhost: simplify async IO vectors

IO vectors implementation is unnecessarily complex, mixing
source and destinations vectors in the same array.

This patch declares two arrays, one for the source and one
for the destination. It also gets rid of seg_awaits variable
in both packed and split implementation, which is the same
as iovec_idx.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Jiayu Hu <jiayu.hu@intel.com>