rpc: Correct the wrong payload size checking

rpc: Correct the wrong payload size checking

<...>
/* Size of message length field. Not counted in VIR_NET_MESSAGE_MAX
 * and VIR_NET_MESSAGE_INITIAL.
 */
const VIR_NET_MESSAGE_LEN_MAX = 4;
</...>

However, msg->bufferLength includes the length word. The wrong checking
was introduced by commit e914dcfd.

* src/rpc/virnetmessage.c:
  - Correct the checking in virNetMessageEncodePayloadRaw
  - Use a new variable to track the new payload length in
    virNetMessageEncodePayloadRaw