spapr_events: re-use EPOW event infrastructure for hotplug events

This extends the data structures currently used to report EPOW events to
guests via the check-exception RTAS interfaces to also include event types
for hotplug/unplug events.

This is currently undocumented and being finalized for inclusion in PAPR
specification, but we implement this here as an extension for guest
userspace tools to implement (existing guest kernels simply log these
events via a sysfs interface that's read by rtas_errd, and current
versions of rtas_errd/powerpc-utils already support the use of this
mechanism for initiating hotplug operations).

We also add support for queues of pending RTAS events, since in the
case of hotplug there's chance for multiple events being in-flight
at any point in time.

Signed-off-by: Nathan Fontenot <nfont@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>
