vhost: introduce specific iovec structure

vhost: introduce specific iovec structure

This patch introduces rte_vhost_iovec struct that contains
both source and destination addresses since we always have
a 1:1 mapping between source and destination. While using
the standard iovec struct might have seemed better, having
to duplicate IO vectors and its iterators is memory
inefficient and make the implementation more complex.

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>