dbus: work with older dbus

dbus 1.2.24 (on RHEL 6) lacks DBUS_TYPE_UNIX_FD; but as we aren't
trying to pass one of those anyways, we can just drop support for
it in our wrapper.  Solves this build error introduced in commit
834c9c94:

  CC     libvirt_util_la-virdbus.lo
util/virdbus.c:242: error: 'DBUS_TYPE_UNIX_FD' undeclared here (not in a function)

* src/util/virdbus.c (virDBusBasicTypes): Drop support for unix fds.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/virdbus.c

 /*
  * virdbus.c: helper for using DBus
  *
- * Copyright (C) 2012 Red Hat, Inc.
+ * Copyright (C) 2012-2013 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -239,7 +239,6 @@ static const char virDBusBasicTypes[] = {
     DBUS_TYPE_STRING,
     DBUS_TYPE_OBJECT_PATH,
     DBUS_TYPE_SIGNATURE,
-    DBUS_TYPE_UNIX_FD
 };
 
 static bool virDBusIsBasicType(char c) {
@@ -1024,6 +1023,9 @@ int virDBusMessageDecode(DBusMessage* msg,
  * '{' - start of a dictionary entry (pair of types)
  * '}' - start of a dictionary entry (pair of types)
  *
+ * At this time, there is no support for Unix fd's ('h'), which only
+ * newer DBus supports.
+ *
  * Passing values in variadic args for basic types is
  * simple, the value is just passed directly using the
  * corresponding C type listed against the type code