s390x/kvm: make flic play well with old kernels

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>

hw/intc/s390_flic_kvm.c

@@ -170,7 +170,8 @@ static int kvm_s390_register_io_adapter(S390FLICState *fs, uint32_t id,
     };
 
     if (!kvm_check_extension(kvm_state, KVM_CAP_IRQ_ROUTING)) {
-        return -ENOSYS;
+        /* nothing to do */
+        return 0;
     }
 
     r = ioctl(flic->fd, KVM_SET_DEVICE_ATTR, &attr);
@@ -195,7 +196,8 @@ static int kvm_s390_io_adapter_map(S390FLICState *fs, uint32_t id,
     int r;
 
     if (!kvm_check_extension(kvm_state, KVM_CAP_IRQ_ROUTING)) {
-        return -ENOSYS;
+        /* nothing to do */
+        return 0;
     }
 
     r = ioctl(flic->fd, KVM_SET_DEVICE_ATTR, &attr);