lttng: move log level name to numerical value conversions to a common util

Move the utils used by the `enable-event` command to convert log level
names (textual) to their values (entries in their respective enums).

Since these utils will be used by other commands in the future, the code
is cleaned-up, notably to maintain type-safety by returning specific
enums rather than casting to `int`.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I3f655bed6cd5b12cae1e93421f9d1c41ab1b6247