memory: use bit 2 for migration

For historical reasons it was bit 3.  Once there, create a constant to
know the number of clients.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Orit Wasserman <owasserm@redhat.com>

include/exec/memory.h

@@ -35,7 +35,8 @@ typedef struct MemoryRegionMmio MemoryRegionMmio;
 
 #define DIRTY_MEMORY_VGA       0
 #define DIRTY_MEMORY_CODE      1
-#define DIRTY_MEMORY_MIGRATION 3
+#define DIRTY_MEMORY_MIGRATION 2
+#define DIRTY_MEMORY_NUM       3        /* num of dirty bits */
 
 struct MemoryRegionMmio {
     CPUReadMemoryFunc *read[3];