spapr_events: add support for dedicated hotplug event source

Hotplug events were previously delivered using an EPOW interrupt
and were queued by linux guests into a circular buffer. For traditional
EPOW events like shutdown/resets, this isn't an issue, but for hotplug
events there are cases where this buffer can be exhausted, resulting
in the loss of hotplug events, resets, etc.

Newer-style hotplug event are delivered using a dedicated event source.
We enable this in supported guests by adding standard an additional
event source in the guest device-tree via /event-sources, and, if
the guest advertises support for the newer-style hotplug events,
using the corresponding interrupt to signal the available of
hotplug/unplug events.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>