ppc/xics: extend the QOM interface to handle ICPs

ppc/xics: extend the QOM interface to handle ICPs

Let's add two new handlers for ICPs. One is to get an ICP object from
a server number and a second is to resend the irqs when needed.

The icp_resend() handler is a temporary workaround needed by the
ics-simple post_load() handler. It will be removed when the post_load
portion can be done at the machine level.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>