g_canonicalize_filename was not introduced until glib 2.58
so we need a temporary backport of its impl.
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>