kvm tools: powerpc: Fix buglet in xics_init() handling of nrcpus

In xics_init() we set the maximum server to kvm->nrcpus, and then set
the nr_servers using maximum server + 1.

That is off by one, in the harmless direction.

Simplify it to just set nr_servers = kvm->nrcpus.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Pekka Enberg <penberg@kernel.org>