migration: fix detached migration with fd Migration with fd uses s->mon to pass the fd. But we only assign the s->mon for !detached migration. Fix it. Once there add a comment indicating that s->mon has two uses. Bug reported by: Wen Congyang <wency@cn.fujitsu.com> Signed-off-by: Juan Quintela <quintela@redhat.com> CC: Wen Congyang <wency@cn.fujitsu.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>