exec: change ramlist from MRU order to a 1-item cache

Most of the time, only 2 items will be active (from/to for a string operation,
or code/data).  But TCG guests likely won't have gigabytes of memory, so
this actually goes down to 1 item.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/exec/cpu-all.h

@@ -495,6 +495,7 @@ typedef struct RAMBlock {
 
 typedef struct RAMList {
     uint8_t *phys_dirty;
+    RAMBlock *mru_block;
     QLIST_HEAD(, RAMBlock) blocks;
 } RAMList;
 extern RAMList ram_list;