doc: document that the monitor console is a privileged control interface

A supposed exploit of QEMU was recently announced as CVE-2019-12928
claiming that the monitor console was insecure because the "migrate"
command enabled arbitrary command execution for a remote attacker.

To be a security risk the user launching QEMU must have configured
the monitor in a way that allows for other users to access it. The
exploit report quoted use of the "tcp" character device backend for
QMP.

This would indeed allow any network user to connect to QEMU and
execute arbitrary commands, however, this is not a flaw in QEMU.
It is the normal expected behaviour of the monitor console and the
commands it supports. Given a monitor connection, there are many
ways to access host file system content besides the migrate command.

The reality is that the monitor console (whether QMP or HMP) is
considered a privileged interface to QEMU and as such must only
be made available to trusted users. IOW, making it available with
no authentication over TCP is simply a, very serious, user
configuration error not a security flaw in QEMU itself.

The one thing this bogus security report highlights though is that
we have not clearly documented the security implications around the
use of the monitor. Add a few paragraphs of text to the security
docs explaining why the monitor is a privileged interface and making
a recommendation to only use the UNIX socket character device backend.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

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