hmp: disable monitor in preconfig state

Ban it for now, if someone would need it to work early,
one would have to implement checks if HMP command is valid
at preconfig state.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1525423069-61903-5-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>