KVM: take kvm_lock for hardware_disable() during cpu hotplug

In kvm_cpu_hotplug(), only CPU_STARTING case is protected by kvm_lock.
This patch adds missing protection for CPU_DYING case.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>