x86: pvclock: remove pvclock_shadow_time

Originally from Jeremy Fitzhardinge.

We can copy the information directly from "struct pvclock_vcpu_time_info",
remove pvclock_shadow_time.

Reviewed-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>