pvops: do not notify callers from register_xenstore_notifier

Currently register_xenstore_notifier notifies the caller during the
registration itself if xenstore is believed to be ready. This behaviour
causes problems to PV on HVM guests, in which case callers should be
notified by xenbus_probe only after the platform pci driver is loaded.
We already make sure xenbus_probe is called at the right time, calling
it either from device_initcall (PV case) or from the platform pci
driver initialization (HVM case) so we don't need this additional
notification.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>