docs: provide documentation on the POWER9 XIVE interrupt controller

This documents the overall XIVE architecture and the XIVE support for
sPAPR guest machines (pseries).

It also provides documentation on the 'info pic' command.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190521082411.24719-1-clg@kaod.org>
Reviewed-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>