Don't start a nested job in qemuMigrationPrepareAny

This nested job is canceled by the first ExitMonitor call (even though
it was not created by the corresponding EnterMonitor call), and
again in qemuMigrationPrepareAny if qemuProcessStart failed.
This can lead to a crash if the vm object was disposed of before calling
qemuDomainRemoveInactive:
0  ..62bc in virClassIsDerivedFrom (klass=0xdeadbeef,
   parent=0x7ffce4cdd270) at util/virobject.c:166
1 ..6666 in virObjectIsClass at util/virobject.c:362
2 ..66b4 in virObjectLock at util/virobject.c:314
3 ..477e in virDomainObjListRemove at conf/domain_conf.c:2359
4 ..7a64 in qemuDomainRemoveInactive at qemu/qemu_domain.c:2087
5 ..956c in qemuMigrationPrepareAny at qemu/qemu_migration.c:2469

This was added by commit e4e28220, exposed by 5a4c2374 and c7ac2519.

https://bugzilla.redhat.com/show_bug.cgi?id=1018267

src/qemu/qemu_migration.c

@@ -2358,10 +2358,6 @@ qemuMigrationPrepareAny(virQEMUDriverPtr driver,
         goto endjob;
     }
 
-    if (qemuDomainObjBeginNestedJob(driver, vm,
-                                    QEMU_ASYNC_JOB_MIGRATION_IN) < 0)
-        goto endjob;
-
     /* Start the QEMU daemon, with the same command-line arguments plus
      * -incoming $migrateFrom
      */
@@ -2370,8 +2366,6 @@ qemuMigrationPrepareAny(virQEMUDriverPtr driver,
                          VIR_QEMU_PROCESS_START_PAUSED |
                          VIR_QEMU_PROCESS_START_AUTODESTROY) < 0) {
         virDomainAuditStart(vm, "migrated", false);
-        if (!qemuDomainObjEndJob(driver, vm))
-            vm = NULL;
         goto endjob;
     }
 
@@ -2474,7 +2468,7 @@ stop:
     qemuProcessStop(driver, vm, VIR_DOMAIN_SHUTOFF_FAILED, 0);
 
 endjob:
-    if (vm && !qemuMigrationJobFinish(driver, vm)) {
+    if (!qemuMigrationJobFinish(driver, vm)) {
         vm = NULL;
     }
     goto cleanup;