x86-64: refine the XSA-9 fix

x86-64: refine the XSA-9 fix

Our product management wasn't happy with the "solution" for XSA-9, and
demanded that customer systems must continue to boot. Rather than
having our and perhaps other distros carry non-trivial patches, allow
for more fine grained control (panic on boot, deny guest creation, or
merely warn) by means of a single line change.

Also, as this was found to be a problem with remotely managed systems,
don't default to boot denial (just deny guest creation).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>