Maintain the number of dirty pages

Calculate the number of dirty pages takes a lot on hosts with lots
of memory.  Just maintain how many pages are dirty.

Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/exec-obsolete.h b/exec-obsolete.h
index f8ffce6..c099256 100644
--- a/exec-obsolete.h
+++ b/exec-obsolete.h
@@ -74,6 +74,11 @@
 static inline int cpu_physical_memory_set_dirty_flags(ram_addr_t addr,
                                                       int dirty_flags)
 {
+    if ((dirty_flags & MIGRATION_DIRTY_FLAG) &&
+        !cpu_physical_memory_get_dirty(addr, TARGET_PAGE_SIZE,
+                                       MIGRATION_DIRTY_FLAG)) {
+        ram_list.dirty_pages++;
+    }
     return ram_list.phys_dirty[addr >> TARGET_PAGE_BITS] |= dirty_flags;
 }
 
@@ -87,6 +92,11 @@
 {
     int mask = ~dirty_flags;
 
+    if ((dirty_flags & MIGRATION_DIRTY_FLAG) &&
+        cpu_physical_memory_get_dirty(addr, TARGET_PAGE_SIZE,
+                                      MIGRATION_DIRTY_FLAG)) {
+        ram_list.dirty_pages--;
+    }
     return ram_list.phys_dirty[addr >> TARGET_PAGE_BITS] &= mask;
 }