If we run with an old kernel that does not support KVM_CAP_IRQ_ROUTING,
we don't have to do anything in the ->register_io_adapter and
->io_adapter_map callbacks and therefore should return 0 instead of
-ENOSYS (just as the non-kvm flic does).

This fixes using adapter interrupts when running under an older kernel,
which broke with "s390x: add I/O adapter registration".
Reported-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>