spapr/xive: Convert KVM device fd checks to assert()

spapr/xive: Convert KVM device fd checks to assert()

All callers guard these functions with an xive_in_kernel() helper. Make
it clear that they are only to be called when the KVM XIVE device exists.

Note that the check on xive is dropped in kvmppc_xive_disconnect(). It
really cannot be NULL since it comes from set_active_intc() which only
passes pointers to allocated objects.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <159679994169.876294.11026653581505077112.stgit@bahia.lan>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>