xen/time: Return -ENODEV from xen_get_wallclock()

For any other error sync_cmos_clock() will reschedule itself
every second or so, for no good reason.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>