spapr: fix error reporting in xics_system_init()

If the user explicitely asked for kernel-irqchip support and "xics-kvm"
initialization fails, we shouldn't fallback to emulated "xics" as we
do now. It is also awkward to print an error message when we have an
errp pointer argument.

Let's use the errp argument to report the error and let the caller decide.
This simplifies the code as we don't need a local Error * here.

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