hvm: Allow triple fault to imply crash rather than reboot

hvm: Allow triple fault to imply crash rather than reboot

While the triple fault action on native hardware will result in a system
reset, any modern operating system can and will make use of less violent
reboot methods.  As a result, the most likely cause of a triple fault is a
fatal software bug.

This patch allows the toolstack to indicate that a triple fault should mean a
crash rather than a reboot.  The default of reboot still remains the same.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Committed-by: Jan Beulich <jbeulich@suse.com>