cpu: Introduce VMSTATE_CPU() macro for CPUState

To be used to embed common CPU state into CPU subclasses.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/exec.c b/exec.c
index f99041b..0b172b4 100644
--- a/exec.c
+++ b/exec.c
@@ -330,7 +330,7 @@
     return 0;
 }
 
-static const VMStateDescription vmstate_cpu_common = {
+const VMStateDescription vmstate_cpu_common = {
     .name = "cpu_common",
     .version_id = 1,
     .minimum_version_id = 1,
@@ -342,8 +342,7 @@
         VMSTATE_END_OF_LIST()
     }
 };
-#else
-#define vmstate_cpu_common vmstate_dummy
+
 #endif
 
 CPUState *qemu_get_cpu(int index)