lib/channel_helper: use bytes for the io_vector
lib/channel_helper: use bytes for the io_vector

Lwt old-style io_vectors are still using strings, this was an oversight,
and there is a breaking change on that interface planned for lwt 5.0.0.
By using bytes and an unsafe conversion to string, we make sure that the
compiler does not violate immutability assumptions.

Signed-off-by: Marcello Seri <34fd14c67154e888f6587d5c0e7eadd13aaf7ca7@citrix.com>

xapi-project/message-switch,xapi-project/message-switch,xapi-project/message-switch