KVM: s390: document KVM_CAP_S390_IRQCHIP

Let's document that this is a capability that may be enabled per-vm.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>