docs/system/security: Restrict "virtualization use case" to specific machines

docs/system/security: Restrict "virtualization use case" to specific machines

Currently our security policy defines a "virtualization use case"
where we consider bugs to be security issues, and a
"non-virtualization use case" where we do not make any security
guarantees and don't consider bugs to be security issues.

The rationale for this split is that much code in QEMU is older and
was not written with malicious guests in mind, and we don't have the
resources to audit, fix and defend it.  So instead we inform users
about what the can in practice rely on as a security barrier, and
what they can't.

We don't currently restrict the "virtualization use case" to any
particular set of machine types.  This means that we have effectively
barred ourselves from adding KVM support to any machine type that we
don't want to put into the "bugs are security issues" category, even
if it would be useful for users to be able to get better performance
with a trusted guest by enabling KVM. This seems an unnecessary
restriction, and in practice the set of machine types it makes
sense to use for untrusted-guest virtualization is quite small.

Specifically, we would like to be able to enable the use of
KVM with the imx8 development board machine types, but we don't
want to commit ourselves to having to support those SoC models
and device models as part of QEMU's security boundary:
https://lore.kernel.org/qemu-devel/20250629204851.1778-3-shentey@gmail.com/

This patch updates the security policy to explicitly list the
machine types we consider to be useful for the "virtualization
use case".

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Message-id: 20251016131159.750480-1-peter.maydell@linaro.org
Acked-by: Markus Armbruster <armbru@redhat.com>