qemudDomainSetVcpus: avoid NULL-deref on failed uuid look-up

* src/qemu/qemu_driver.c (qemudDomainSetVcpus): Upon look-up failure,
i.e., vm==NULL, goto cleanup, rather than to "endjob", superficially
since the latter would dereference vm, but more fundamentally because
we certainly don't want to call qemuDomainObjEndJob before we've
even attempted qemuDomainObjBeginJob.

src/qemu/qemu_driver.c

@@ -5530,7 +5530,7 @@ static int qemudDomainSetVcpus(virDomainPtr dom, unsigned int nvcpus) {
         virUUIDFormat(dom->uuid, uuidstr);
         qemuReportError(VIR_ERR_NO_DOMAIN,
                         _("no domain with matching uuid '%s'"), uuidstr);
-        goto endjob;
+        goto cleanup;
     }
 
     if (qemuDomainObjBeginJob(vm) < 0)