Fix save/restore for HVM domains with viridian=1

Fix save/restore for HVM domains with viridian=1

xc_domain_save/restore currently pay no attention to
HVM_PARAM_VIRIDIAN which results in an HVM domain running a recent
version on Windows (post-Vista) locking up on a domain restore due to
EOIs (done via a viridian MSR write) being silently dropped.  This
patch adds an extra save entry for the viridian parameter and also
adds code in the viridian kernel module to catch attempted use of
viridian functionality when the HVM parameter has not been set.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>