KVM: arm/arm64: kvm_arch_destroy_vm cleanups

kvm_vgic_vcpu_destroy already gets called from kvm_vgic_destroy for
each vcpu, so we don't have to call it from kvm_arch_vcpu_free.

Additionally the other architectures set kvm->online_vcpus to zero
after freeing them. We might as well do that for ARM too.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>