Remove warnings when using bt_plugin_create_all_from_dir()

Remove warnings when using bt_plugin_create_all_from_dir()

bt_plugin_create_all_from_dir() expects an existing directory, otherwise
it fails with a WARN-level log message because it's the user's
responsibility to ensure this.

A few functions of the library and the CLI call this with nonexisting
paths, resulting in warnings under normal usage (for example, default
plugin search paths which do not exist; this is not an error nor a
warning).

This patch fixes this by testing the existence of the paths passed to
bt_plugin_create_all_from_dir() with g_file_test(), skipping (usually
with a VERBOSE-level message) when it's not the case.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>