Set last_sent_block

In a82d593b61 I accidentally removed the setting of
last_sent_block,  put it back.

Symptoms:
  Multithreaded compression only uses one thread.
  Migration is a bit less efficient since it won't use 'cont' flags.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Fixes: a82d593b61
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/ram.c

@@ -1249,6 +1249,7 @@ static int ram_save_target_page(MigrationState *ms, QEMUFile *f,
         if (unsentmap) {
             clear_bit(dirty_ram_abs >> TARGET_PAGE_BITS, unsentmap);
         }
+        last_sent_block = block;
     }
 
     return res;