daemon: Use prog_exists to check for features.

This updates commit 0938509e04.

daemon/tar.c

@@ -1,5 +1,5 @@
 /* libguestfs - the guestfsd daemon
- * Copyright (C) 2009 Red Hat Inc.
+ * Copyright (C) 2009-2010 Red Hat Inc.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -33,8 +33,7 @@
 int
 optgroup_xz_available (void)
 {
-  int r = access ("/usr/bin/xz", X_OK);
-  return r == 0;
+  return prog_exists ("xz");
 }
 
 /* Redirect errors from the tar command to the error file, then