tests: Use real size for iov tests

We were using -1 instead of the real size because the functions check
what is bigger, size in bytes or the size of the iov.  Recent gcc's
barf at this.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
--

Remove comments about this feature.
Fix missing -1.
