KVM: Make kvm host use the paravirt clocksource structs

This patch updates the kvm host code to use the pvclock structs.
It also makes the paravirt clock compatible with Xen.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>