migration: Add dirty_pages_rate to query migrate output

It indicates how many pages were dirtied during the last second.

Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/hmp.c b/hmp.c
index 96e2174..2b97982 100644
--- a/hmp.c
+++ b/hmp.c
@@ -175,6 +175,10 @@
                        info->ram->normal);
         monitor_printf(mon, "normal bytes: %" PRIu64 " kbytes\n",
                        info->ram->normal_bytes >> 10);
+        if (info->ram->dirty_pages_rate) {
+            monitor_printf(mon, "dirty pages rate: %" PRIu64 " pages\n",
+                           info->ram->dirty_pages_rate);
+        }
     }
 
     if (info->has_disk) {