KVM: PPC: Book3S: ifdef on CONFIG_KVM_BOOK3S_32_HANDLER for 32bit

The book3s_32 target can get built as module which means we don't see the
config define for it in code. Instead, check on the bool define
CONFIG_KVM_BOOK3S_32_HANDLER whenever we want to know whether we're building
for a book3s_32 host.

This fixes running book3s_32 kvm as a module for me.

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