ptp_kvm: probe for kvm guest availability

In the event of moving pvclock_pvti_cpu0_va() definition to common
pvclock code, this function would return a value on non KVM guests.
Later on this would fail with a GPF on ptp_kvm_init when running on a
Xen guest. Therefore, ptp_kvm_init() should check whether it is running
in a KVM guest.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Acked-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>