qemu: Resolve Coverity BAD_SIZEOF

Coverity complains about the calculation of the buf & len within
the PROBE macro.  So to quiet things down, do the calculation prior
to usage in either write() or qemuMonitorIOWriteWithFD() calls and
then have the PROBE use the calculated values - which works.

src/qemu/qemu_monitor.c

@@ -478,6 +478,8 @@ static int
 qemuMonitorIOWrite(qemuMonitorPtr mon)
 {
     int done;
+    char *buf;
+    size_t len;
 
     /* If no active message, or fully transmitted, the no-op */
     if (!mon->msg || mon->msg->txOffset == mon->msg->txLength)
@@ -489,22 +491,16 @@ qemuMonitorIOWrite(qemuMonitorPtr mon)
         return -1;
     }
 
+    buf = mon->msg->txBuffer + mon->msg->txOffset;
+    len = mon->msg->txLength - mon->msg->txOffset;
     if (mon->msg->txFD == -1)
-        done = write(mon->fd,
-                     mon->msg->txBuffer + mon->msg->txOffset,
-                     mon->msg->txLength - mon->msg->txOffset);
+        done = write(mon->fd, buf, len);
     else
-        done = qemuMonitorIOWriteWithFD(mon,
-                                        mon->msg->txBuffer + mon->msg->txOffset,
-                                        mon->msg->txLength - mon->msg->txOffset,
-                                        mon->msg->txFD);
+        done = qemuMonitorIOWriteWithFD(mon, buf, len, mon->msg->txFD);
 
     PROBE(QEMU_MONITOR_IO_WRITE,
-          "mon=%p buf=%s len=%d ret=%d errno=%d",
-          mon,
-          mon->msg->txBuffer + mon->msg->txOffset,
-          mon->msg->txLength - mon->msg->txOffset,
-          done, errno);
+          "mon=%p buf=%s len=%lu ret=%d errno=%d",
+          mon, buf, len, done, errno);
 
     if (mon->msg->txFD != -1) {
         PROBE(QEMU_MONITOR_IO_SEND_FD,