s390x/css: Factor out virtual css bridge and bus

Currently, common base layers virtual css bridge and bus are
defined in hw/s390x/virtio-ccw.c(h). In order to support
multiple types of devices in the virtual channel subsystem,
especially non virtio-ccw, refactoring work needs to be done.

This work is just a pure code move without any functional change
except dropping an empty function virtual_css_bridge_init() and
virtio_ccw_busdev_unplug() changing. virtio_ccw_busdev_unplug()
is specific to virtio-ccw but gets referenced from the common
virtual css bridge code. To keep the functional changes to a
minimum we export this function from virtio-ccw.c and continue
to reference it inside virtual_css_bridge_class_init()
(now living in hw/s390x/css-bridge.c). A follow-up patch will
clean this up.

Signed-off-by: Jing Liu <liujbjl@linux.vnet.ibm.com>
Reviewed-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

include/hw/s390x/css-bridge.h

@@ -0,0 +1,31 @@
+/*
+ * virtual css bridge definition
+ *
+ * Copyright 2012,2016 IBM Corp.
+ * Author(s): Cornelia Huck <cornelia.huck@de.ibm.com>
+ *            Pierre Morel <pmorel@linux.vnet.ibm.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or (at
+ * your option) any later version. See the COPYING file in the top-level
+ * directory.
+ */
+
+#ifndef HW_S390X_CSS_BRIDGE_H
+#define HW_S390X_CSS_BRIDGE_H
+#include "qom/object.h"
+#include "hw/qdev-core.h"
+
+/* virtual css bridge */
+#define TYPE_VIRTUAL_CSS_BRIDGE "virtual-css-bridge"
+
+/* virtual css bus type */
+typedef struct VirtualCssBus {
+    BusState parent_obj;
+} VirtualCssBus;
+
+#define TYPE_VIRTUAL_CSS_BUS "virtual-css-bus"
+#define VIRTUAL_CSS_BUS(obj) \
+     OBJECT_CHECK(VirtualCssBus, (obj), TYPE_VIRTUAL_CSS_BUS)
+VirtualCssBus *virtual_css_bus_init(void);
+
+#endif