spapr: ensure we have at least one XICS server

spapr: ensure we have at least one XICS server

XICS needs to know the upper value for cpu_index as it is used to compute
the number of servers:

    smp_cpus * kvmppc_smt_threads() / smp_threads

When passing -smp cpus=1,threads=9 on a POWER8 host, we end up with:

    1 * 8 / 9 = 0

... which leads to an assertion in both emulated:

Number of servers needs to be greater 0
Aborted (core dumped)

... and in-kernel XICS:

xics_kvm_realize: Assertion `icp->nr_servers' failed.
Aborted (core dumped)

With this patch, we are sure that nr_servers > 0. Passing the same bogus
-smp option then leads to:

qemu-system-ppc64: Cannot support more than 8 threads on PPC with KVM

... which is a lot more explicit than the XICS errors.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>