KVM: PPC: Add kvm_book3s_64.h

In the process of generalizing as much code as possible, I also moved
the shadow vcpu code together to a generic book3s file. Unfortunately
the location of the shadow vcpu is different on 32 and 64 bit, so we
need a wrapper function to tell us where it is.

That sounded like a perfect fit for a subarch specific header file.
Here we can put anything that needs to be different between those two.

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