The canonicalize_file_name(path) is equivalent to calling
realpath(path, NULL). Passing NULL for the second arg of
realpath is not standardized behaviour, however, Linux,
FreeBSD > 6.4 and macOS > 10.5 all support this critical
extension.

This leaves Windows which doesn't provide realpath at all.
The g_canonicalize_filename() function doesn't expand
symlinks, so is not strictly equivalent to realpath()
but is close enough for our Windows portability needs
right now.
Reviewed-by: NFabiano Fidêncio <fidencio@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>