KVM: PPC: Name generic 64-bit code generic

We have quite some code that can be used by Book3S_32 and Book3S_64 alike,
so let's call it "Book3S" instead of "Book3S_64", so we can later on
use it from the 32 bit port too.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>