KVM: Bump KVM_MAX_IRQ_ROUTES for s390

The maximum number for irq routes is currently 1024, which is a bit on
the small size for s390: We support up to 4 x 64k virtual devices with
up to 64 queues, and we need one route for each of the queues if we want
to operate it via irqfd.

Let's bump this to 4k on s390 for now, as this at least covers the saner
setups.

We need to find a more general solution, though, as we can't just grow
the routing table indefinitly.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>