Fix: lttng-live data_size and version endianness

The data_size and cmd_version fields need to be sent in network byte
order to the relay. These fields are currently ignored by the relay
because the commands have a fix-sized payload so it does not cause any
problem for now, but they may be used in the future, so we should avoid
any confusion.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>