extending xio_vmsg to support array of data iovecs and also to support
allocated pointer of iovecs.
the array is limited to 4 elements. if application wishes to use more it should
1. use xio_set_opt to set the maximum in/out iovec size.
2. allocate the pointer portion and use it instead of the array.

Signed-off-by: Eyal Salomon <esalomon@mellanox.com>