KVM: Ensure all vcpus are consistent with in-kernel irqchip settings

(cherry picked from commit 3e515705a1f46beb1c942bb8043c16f8ac7b1e9e)

If some vcpus are created before KVM_CREATE_IRQCHIP, then
irqchip_in_kernel() and vcpu->arch.apic will be inconsistent, leading
to potential NULL pointer dereferences.

Fix by:
- ensuring that no vcpus are installed when KVM_CREATE_IRQCHIP is called
- ensuring that a vcpu has an apic if it is installed after KVM_CREATE_IRQCHIP

This is somewhat long winded because vcpu->arch.apic is created without
kvm->lock held.

Based on earlier patch by Michael Ellerman.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
static void kvm_on_user_return(struct user_return_notifier *urn)
{
	unsigned slot;
	struct kvm_shared_msrs *locals
		= container_of(urn, struct kvm_shared_msrs, urn);
	struct kvm_shared_msr_values *values;

	for (slot = 0; slot < shared_msrs_global.nr; ++slot) {
		values = &locals->values[slot];
		if (values->host != values->curr) {
			wrmsrl(shared_msrs_global.msrs[slot], values->host);
			values->curr = values->host;
		}
	}
	locals->registered = false;
	user_return_notifier_unregister(urn);
}

static void kvm_on_user_return(struct user_return_notifier *urn)
{
	unsigned slot;
	struct kvm_shared_msrs *locals
		= container_of(urn, struct kvm_shared_msrs, urn);
	struct kvm_shared_msr_values *values;

	for (slot = 0; slot < shared_msrs_global.nr; ++slot) {
		values = &locals->values[slot];
		if (values->host != values->curr) {
			wrmsrl(shared_msrs_global.msrs[slot], values->host);
			values->curr = values->host;
		}
	}
	locals->registered = false;
	user_return_notifier_unregister(urn);
}
