KVM: PPC: Allocate vcpu struct using vmalloc

We used to use get_free_pages to allocate our vcpu struct. Unfortunately
that call failed on me several times after my machine had a big enough
uptime, as memory became too fragmented by then.

Fortunately, we don't need it to be page aligned any more! We can just
vmalloc it and everything's great.

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