KVM: PPC: Book3S: Enable in-kernel XICS emulation by default

The in-kernel XICS emulation is faster than doing it all in QEMU
and it has got a lot of testing, so enable it by default.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>