x86: kvmclock: add flag to indicate pvclock counts from zero

Setting sched clock stable for kvmclock causes the printk timestamps
to not start from zero, which is different from baremetal and
can possibly break userspace. Add a flag to indicate that
hypervisor sets clock base at zero when kvmclock is initialized.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>