Efficiently restore Accelio larger iovec arrays.

Pool allocating a custom iovec array was up to 35% slower than
builtin, so this version introduces a widened xio_msg wrapper
and uses the wide structure in both the expanded leader structure
and the optional trailer array.

Tested with XIO_MSGR_IOVLEN of 4, to verify chaining at msg_cnt ==
2, then defined to 16 (the original XIO_IOVMAX).

Signed-off-by: Matt Benjamin <matt@linuxbox.com>