KVM: use rcu access function for irq routing

irq routing is rcu protected. Use the proper access functions.
Found by sparse

virt/kvm/irqchip.c:233:13: warning: incorrect type in assignment (different address spaces)
virt/kvm/irqchip.c:233:13:    expected struct kvm_irq_routing_table *old
virt/kvm/irqchip.c:233:13:    got struct kvm_irq_routing_table [noderef] <asn:4>*irq_routing

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>