monitor: let mon_list be tail queue

It was QLIST.  I want to use this list to do monitor priority job later,
which need tail insertion ability.  So switching to a tail queue.

Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-9-peterx@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>