PPC: sPAPR: Only use getpagesize() when we run with kvm

We currently size the msi window trap page according to the host's page
size so that we poke a working hole into a memory slot in case we overlap.

However, this is only ever necessary with KVM active. Without KVM, we should
rather try to be host platform agnostic and use a constant size: 4k.

This fixes a build breakage on win32 hosts.

Signed-off-by: Alexander Graf <agraf@suse.de>