Fix performance regression in qemu_get_ram_ptr

When the commit f471a17e converted the
ram_blocks structure to QLIST, it also removed the conditional check before
switching the current block at the beginning of the list.

In the common use case where ram_blocks has a few blocks with only one
frequently accessed (the main RAM), this has a performance impact as it
performs the useless list operations on each call (which are on a really
hot path).

On my machine emulation (ARM on amd64), this patch reduces the
percentage of CPU time spent in qemu_get_ram_ptr from 6.3% to 2.1% in the
profiling of a full boot.
Signed-off-by: NVincent Palatin <vpalatin@chromium.org>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

exec.c

@@ -2957,8 +2957,11 @@ void *qemu_get_ram_ptr(ram_addr_t addr)
 
     QLIST_FOREACH(block, &ram_list.blocks, next) {
         if (addr - block->offset < block->length) {
+            /* Move this entry to to start of the list.  */
+            if (block != QLIST_FIRST(&ram_list.blocks)) {
                 QLIST_REMOVE(block, next);
                 QLIST_INSERT_HEAD(&ram_list.blocks, block, next);
+            }
             return block->host + (addr - block->offset);
         }
     }