kvm-irqchip: only commit route when irqchip is used

Reported from Alexey Kardashevskiy:

3f1fea0fb5bf "kvm-irqchip: do explicit commit when update irq" produces
a crash on pseries guest running with VFIO on POWER8 machine as it does
not support KVM_CAP_IRQCHIP (KVM_CAP_IRQ_XICS is there instead). At the
result, KVMState::irq_routes is NULL when VFIO calls
kvm_irqchip_commit_routes.

This makes the routing update conditional.

Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
