migration: print total downtime for final phase of migration

Signed-off-by: Juan Quintela <quintela@redhat.com>

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Orit Wasserman <owasserm@redhat.com>
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 2f8477e..4686057 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -2304,6 +2304,8 @@
 - "total-time": total amount of ms since migration started.  If
                 migration has ended, it returns the total migration
 		 time (json-int)
+- "downtime": only present when migration has finished correctly
+              total amount in ms for downtime that happened (json-int)
 - "ram": only present if "status" is "active", it is a json-object with the
   following RAM information (in bytes):
          - "transferred": amount transferred (json-int)
@@ -2341,6 +2343,7 @@
           "remaining":123,
           "total":246,
           "total-time":12345,
+          "downtime":12345,
           "duplicate":123,
           "normal":123,
           "normal-bytes":123456