Start to use dedicate IO thread for QMP monitors that are not using
MUXed chardev.

Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-21-peterx@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>