Ensure RPC message is cleared before being reused

Ensure RPC message is cleared before being reused

To save on memory reallocation, virNetMessage instances that
have been transmitted, may be reused for a subsequent incoming
message. We forgot to clear out the old data of the message
fully, which caused later confusion upon read.

* src/rpc/virnetserverclient.c: memset entire message before
  reusing it