gio-tool-mount: Allow mounting by the given UUID
"gio mount" only allows mounting volumes using device files, but not using UUIDs. Volume monitors for various GVfs locations usually supports just UUIDs. It would be handy to support them also for testing purposes (because "g_file_mount_enclosing_volume()" does something else than "g_volume_mount()" in case of shares provided by GOA volume monitor for example). Let's update "-d" option to support also UUIDs.