rpc: improve error message for bounds check

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>