Users might want to enable all features, without realizing some
features have negative effect. Mention the TCI feature is slow
and experimental, hoping it will be selected knowingly.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210125144530.2837481-3-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>