rpc: improve error message for bounds check
    
    If we exceed a fixed limit in RPC code we get a horrible message
    like this, if the parameter type is a 'string', because we forgot
    to initialize the error message type field:
    
      $ virsh snapshot-list ostack1
      error: too many remote undefineds: 1329 > 1024
    
    It would also be useful to know which RPC call and field was
    exceeded. So this patch makes us report:
    
      $ virsh snapshot-list ostack1
      error: too many remote undefineds: 1329 > 1024,
      in parameter 'names' for 'virDomainSnapshotListNames'
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>