xics: Create sPAPR specific ICS subtype

xics: Create sPAPR specific ICS subtype

We create a subtype of TYPE_ICS specifically for sPAPR.  For now all this
does is move the setup of the PAPR specific hcalls and RTAS calls to
the realize() function for this, rather than requiring the PAPR code to
explicitly call xics_spapr_init().  In future it will have some more
function.

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