qemu: Add check for invalid iothread_id in qemuDomainChgIOThread

https://bugzilla.redhat.com/show_bug.cgi?id=1251886

Since iothread_id == 0 is an invalid value for QEMU let's point
that out specifically.  For the IOThreadDel code, the failure would
have ended up being a failure to find the IOThread ID; however, for
the IOThreadAdd code - an IOThread 0 was added and that isn't good.

It seems during many reviews/edits to the code the check for
iothread_id = 0 being invalid was lost - it could have originally
been in the API code, but requested to be moved - I cannot remember.

src/qemu/qemu_driver.c

@@ -6134,6 +6134,12 @@ qemuDomainChgIOThread(virQEMUDriverPtr driver,
     virDomainDefPtr persistentDef;
     int ret = -1;
 
+    if (iothread_id == 0) {
+        virReportError(VIR_ERR_INVALID_ARG, "%s",
+                       _("invalid value of 0 for iothread_id"));
+        return -1;
+    }
+
     cfg = virQEMUDriverGetConfig(driver);
 
     priv = vm->privateData;