spapr_events: event-scan RTAS interface

We don't actually rely on this interface to surface hotplug events, and
instead rely on the similar-but-interrupt-driven check-exception RTAS
interface used for EPOW events. However, the existence of this interface
is needed to ensure guest kernels initialize the event-reporting
interfaces which will in turn be used by userspace tools to handle these
events, so we implement this interface here.

Since events surfaced by this call are mutually exclusive to those
surfaced via check-exception, we also update the RTAS event queue code
to accept a boolean to mark/filter for events accordingly.

Events of this sort are not currently generated by QEMU, but the interface
has been tested by surfacing hotplug events via event-scan in place
of check-exception.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
