stubs: Add vm state change handler stubs

They will be used by BlockBackend code in block-obj-y, which doesn't
always get linked with common-obj-y. Add stubs to keep ld happy.
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <20170815130740.31229-2-famz@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

stubs/Makefile.objs

@@ -19,6 +19,7 @@ stub-obj-y += is-daemonized.o
 stub-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
 stub-obj-y += machine-init-done.o
 stub-obj-y += migr-blocker.o
+stub-obj-y += change-state-handler.o
 stub-obj-y += monitor.o
 stub-obj-y += notify-event.o
 stub-obj-y += qtest.o

stubs/change-state-handler.c

+#include "qemu/osdep.h"
+#include "qemu-common.h"
+#include "sysemu/sysemu.h"
+
+VMChangeStateEntry *qemu_add_vm_change_state_handler(VMChangeStateHandler *cb,
+                                                     void *opaque)
+{
+    return NULL;
+}
+
+void qemu_del_vm_change_state_handler(VMChangeStateEntry *e)
+{
+    /* Nothing to do. */
+}