kvm tools: move kvm_cpus into struct kvm

kvm tools: move kvm_cpus into struct kvm

There's no reason the array of guest specific vcpus is global. Move it into
struct kvm.

Also split up arch specific vcpu init from the generic code and call it from
the kvm_cpu initializer.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>