spapr-pci: Convert init() callback to realize()

This converts the old-style init() callback to a new style realize()
callback.

As a part of conversion, this replaces fprintf(stderr) with error_setg()
as realize() does not "return" any value, instead it puts the extended
error into **errp.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Mike Day <ncmike@ncultra.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>