KVM: s390: provide CMMA attributes only if available

Let's not provide the device attribute for cmma enabling and clearing
if the hardware doesn't support it.

This also helps getting rid of the undocumented return value "-EINVAL"
in case CMMA is not available when trying to enable it.

Also properly document the meaning of -EINVAL for CMMA clearing.

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

Documentation/virtual/kvm/api.txt

@@ -2520,6 +2520,7 @@ Parameters: struct kvm_device_attr
 Returns: 0 on success, -1 on error
 Errors:
   ENXIO:  The group or attribute is unknown/unsupported for this device
+          or hardware support is missing.
   EPERM:  The attribute cannot (currently) be accessed this way
           (e.g. read-only attribute, or attribute that only makes
           sense when the device is in a different state)
@@ -2547,6 +2548,7 @@ Parameters: struct kvm_device_attr
 Returns: 0 on success, -1 on error
 Errors:
   ENXIO:  The group or attribute is unknown/unsupported for this device
+          or hardware support is missing.
 
 Tests whether a device supports a particular attribute.  A successful
 return indicates the attribute is implemented.  It does not necessarily