xio: accelio requires a NULL in.header.iov_base

Signed-off-by: Casey Bodley <casey@linuxbox.com>
xio: fix for messages ending on msg boundaries

if the calls to xio_count_buffers() result in an empty last message,
decrement the ex_cnt before allocating the message array.

this is also the case that hits the line:
  req = 0; /* poison.  trap if we try to use it. */
so we have to avoid accessing req to assign msg_iov there as well

Signed-off-by: Casey Bodley <casey@linuxbox.com>