Adjust legacy max payload size to account for header information

Commit 27e81517 set the payload size to 256 KB, which is
actually the max packet size, including the size of the header.

Reduce this by VIR_NET_MESSAGE_HEADER_MAX (24) and set
VIR_NET_MESSAGE_LEGACY_PAYLOAD_MAX to 262120, which was the original
value before increasing the limit in commit eb635de1.

(cherry picked from commit 609eb987)

src/rpc/virnetprotocol.x

@@ -55,7 +55,7 @@ const VIR_NET_MESSAGE_INITIAL = 65536;
  * payload size. We need to remember this for compat with
  * old clients.
  */
-const VIR_NET_MESSAGE_LEGACY_PAYLOAD_MAX = 262144;
+const VIR_NET_MESSAGE_LEGACY_PAYLOAD_MAX = 262120;
 
 /* Maximum total message size (serialised). */
 const VIR_NET_MESSAGE_MAX = 16777216;