Optimization: lttng UI uses sprintf instead of strcpy

The LTTng client currently uses sprintf with format strings
that don't contain a format specifier.

Users have reported concerns for the scalability and performance
of the LTTng UI because of this overkill use of sprintf instead
of strcpy which, presumably, could be inlined by the compiler.

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