Sort RAMBlocks by ID for migration, not by ram_addr

ram_addr is (a) unstable (b) going away.  Sort by idstr instead.

Commit b2e0a138 initially introduced the sorting for the purpose
of improving debuggability.  After this patch, the order is still
stable, but perhaps less usable by a human.
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

arch_init.c

@@ -217,12 +217,8 @@ static int block_compar(const void *a, const void *b)
 {
     RAMBlock * const *ablock = a;
     RAMBlock * const *bblock = b;
-    if ((*ablock)->offset < (*bblock)->offset) {
-        return -1;
-    } else if ((*ablock)->offset > (*bblock)->offset) {
-        return 1;
-    }
-    return 0;
+
+    return strcmp((*ablock)->idstr, (*bblock)->idstr);
 }
 
 static void sort_ram_list(void)