rpc: Double buffer size instead of quadrupling buffer size.

When increasing the buffer size up to VIR_NET_MESSAGE_MAX, we
currently quadruple it each time.  This unfortunately means that we
cannot allow certain buffer sizes -- for example the current
VIR_NET_MESSAGE_MAX == 33554432 can never be "hit" since ‘newlen’
jumps from 16MB to 64MB.

Instead of quadrupling, double it each time.

Thanks: Daniel Berrange.
Signed-off-by: NRichard W.M. Jones <rjones@redhat.com>

src/rpc/virnetmessage.c

@@ -358,7 +358,8 @@ int virNetMessageEncodePayload(virNetMessagePtr msg,
 
     /* Try to encode the payload. If the buffer is too small increase it. */
     while (!(*filter)(&xdr, data, 0)) {
-        unsigned int newlen = (msg->bufferLength - VIR_NET_MESSAGE_LEN_MAX) * 4;
+        unsigned int newlen = msg->bufferLength - VIR_NET_MESSAGE_LEN_MAX;
+        newlen *= 2;
 
         if (newlen > VIR_NET_MESSAGE_MAX) {
             virReportError(VIR_ERR_RPC, "%s", _("Unable to encode message payload"));