Checks if calling Variant.get with format_string on value would be valid from a type-compatibility standpoint. format_string is assumed to be a valid format string (from a syntactic standpoint).
If copy_only is TRUE then this function additionally checks that it would be safe to call Variant.unref on value immediately after the call to Variant.get without invalidating the result. This is only possible if deep copies are made (ie: there are no pointers to the data inside of the soon-to-be-freed glib.Variant instance). If this check fails then a g_critical() is printed and FALSE is returned.