KVM: PPC: Book3S PR: Allocate kvm_vcpu structs from kvm_vcpu_cache

This makes PR KVM allocate its kvm_vcpu structs from the kvm_vcpu_cache
rather than having them embedded in the kvmppc_vcpu_book3s struct,
which is allocated with vzalloc.  The reason is to reduce the
differences between PR and HV KVM in order to make is easier to have
them coexist in one kernel binary.

With this, the kvm_vcpu struct has a pointer to the kvmppc_vcpu_book3s
struct.  The pointer to the kvmppc_book3s_shadow_vcpu struct has moved
from the kvmppc_vcpu_book3s struct to the kvm_vcpu struct, and is only
present for 32-bit, since it is only used for 32-bit.

Signed-off-by: Paul Mackerras <paulus@samba.org>
[agraf: squash in compile fix from Aneesh]
Signed-off-by: Alexander Graf <agraf@suse.de>