KVM: s390: document FLIC behavior on unsupported

FLIC behavior deviates from the API documentation in reporting EINVAL
instead of ENXIO for KVM_SET_DEVICE_ATTR/KVM_GET_DEVICE_ATTR when the group
or attribute is unknown/unsupported. Unfortunately this can not be fixed
for historical reasons. Let us at least have it documented.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>