vhost: improve IO vector logic

vhost: improve IO vector logic

IO vectors and their iterators arrays were part of the
async metadata but not their indexes.

In order to makes this more consistent, the patch adds the
indexes to the async metadata. Doing that, we can avoid
triggering DMA transfer within the loop as it IO vector
index overflow is now prevented in the async_mbuf_to_desc()
function.

Note that previous detection mechanism was broken
since the overflow already happened when detected, so OOB
memory access would already have happened.

With this changes done, virtio_dev_rx_async_submit_split()
and virtio_dev_rx_async_submit_packed() can be further
simplified.

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