KVM: PPC: Use the vcpu kmem_cache when allocating new VCPUs

Currently the code kzalloc()s new VCPUs instead of using the kmem_cache
which is created when KVM is initialized.

Modify it to allocate VCPUs from that kmem_cache.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>