Do not use g_quark_from_static_string in unloadable code

Do not use g_quark_from_static_string in unloadable code

The GLib Quark documentation says that g_quark_from_static_string:

   "... can only be used if the string will continue to exist until the
    program terminates. It can be used with statically allocated
    strings in the main program, but not with statically allocated
    memory in dynamically loaded modules."

https://developer.gnome.org/glib/stable/glib-Quarks.html#g-quark-from-static-string

Convert call sites of g_quark_from_static_string to
g_quark_from_string in code that is potentially unloadable. Call
sites in the babeltrace main program remain unchanged.

Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>