KVM: x86: initialize kvmclock_offset

Make the guest's kvmclock count up from zero, not from the host boot
time.  The guest cannot rely on that anyway because it changes on
migration, the numbers are easier on the eye and finally it matches the
desired semantics of the Hyper-V time reference counter.

Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>