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 int vcpu_mmio_read(struct kvm_vcpu *vcpu, gpa_t addr, int len, void *v)
{
	int handled = 0;
	int n;

	do {
		n = min(len, 8);
		if (!(vcpu->arch.apic &&
		      !kvm_iodevice_read(&vcpu->arch.apic->dev, addr, n, v))
		    && kvm_io_bus_read(vcpu->kvm, KVM_MMIO_BUS, addr, n, v))
			break;
		trace_kvm_mmio(KVM_TRACE_MMIO_READ, n, addr, *(u64 *)v);
		handled += n;
		addr += n;
		len -= n;
		v += n;
	} while (len);

	return handled;
}

static int vcpu_mmio_read(struct kvm_vcpu *vcpu, gpa_t addr, int len, void *v)
{
	int handled = 0;
	int n;

	do {
		n = min(len, 8);
		if (!(vcpu->arch.apic &&
		      !kvm_iodevice_read(&vcpu->arch.apic->dev, addr, n, v))
		    && kvm_io_bus_read(vcpu->kvm, KVM_MMIO_BUS, addr, n, v))
			break;
		trace_kvm_mmio(KVM_TRACE_MMIO_READ, n, addr, *(u64 *)v);
		handled += n;
		addr += n;
		len -= n;
		v += n;
	} while (len);

	return handled;
}
