nodedev: let compiler help us on switches
    
    The compiler can alert us to places where we need to expand switch
    statements because we add a new enum value, but only if we don't
    have a default case.
    
    * src/conf/node_device_conf.c (virNodeDeviceDefFormat)
    (virNodeDevCapsDefParseXML, virNodeDevCapsDefFree): Drop default
    case.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>