qemu: Fix job usage in virDomainGetBlockIoTune

CVE-2013-6458

Every API that is going to begin a job should do that before fetching
data from vm->def.

(cherry picked from commit 3b564259)

src/qemu/qemu_driver.c

@@ -14870,12 +14870,6 @@ qemuDomainGetBlockIoTune(virDomainPtr dom,
         goto cleanup;
     }
 
-    device = qemuDiskPathToAlias(vm, disk, NULL);
-
-    if (!device) {
-        goto cleanup;
-    }
-
     if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_MODIFY) < 0)
         goto cleanup;
 
@@ -14883,6 +14877,11 @@ qemuDomainGetBlockIoTune(virDomainPtr dom,
                                         &persistentDef) < 0)
         goto endjob;
 
+    device = qemuDiskPathToAlias(vm, disk, NULL);
+    if (!device) {
+        goto endjob;
+    }
+
     if (flags & VIR_DOMAIN_AFFECT_LIVE) {
         priv = vm->privateData;
         qemuDomainObjEnterMonitor(driver, vm);