Add a single trigger for all diagnostic keyhandlers

Add a single trigger for all diagnostic keyhandlers

Add a new keyhandler that triggers all the side-effect-free
keyhandlers.  This lets automated tests (and users) log the full set
of keyhandlers without having to be aware of which ones might reboot
the host.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>