[HVM] Avoid mis-optimisation of the SVM/VMX exit handlers

[HVM] Avoid mis-optimisation of the SVM/VMX exit handlers
by passing a *pointer* to the register state, instead of
'passing by value'.

This should fix the bugs resulting in display of
"trying to set reserved bit in EFER"
on the Xen console.

Signed-off-by: Keir Fraser <keir@xensource.com>