Fix: missing hostname context

This context adds the hostname context field for the event(s) selected.
It is particularly useful when tracing a machine with containers (lxc),
that way we can easily distinguish events generated inside a container.

The API is extended but no features nor behaviors are changed.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>