lock_iovec will lock an I/O vec and the memory to which it refers and
create a iovec in the host space that refers to it, with full error
unwinding. Add helper_iovec_unlock to unlock the partially locked iovec
in case there's an error. The code will be used in iovec_unlock when
that is committed.

Note: memory handling likely could be rewritten to use q_autofree. That
will be explored in the future since what we have now works well enough.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>