monitor: move skip_flush into monitor_data_init

It's part of the data init.  Collect it.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-6-peterx@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>