Kvm clocksource, new spin

Hi folks,

Here's a new spin of the clocksource implementation.
In this new version:
* followed avi's suggestion of:
- letting the cpu itself register its memory area.
- using a gfn instead of a phys addr as a parameter, to be sure we can cover the whole memory area
- write guest time at exits.

Also, I 'm not using an anonymous struct in the kvm_hv_clock union, so the vcpu struct can grab just what it needs, and not the whole padding the guest needs

This is it.

Have fun