qemu: stop NBD server after successful migration

In qemuMigrationFinish mig->nbd can not be initialized by
qemuMigrationEatCookie without the QEMU_MIGRATION_COOKIE_NBD flag.
That causes qemuMigrationStopNBDServer to return early without
stopping the NBD server properly.
Signed-off-by: NWeiwei Li <nuonuoli@tencent.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
(cherry picked from commit c3012a02)

src/qemu/qemu_migration.c

@@ -4823,7 +4823,8 @@ qemuMigrationFinish(virQEMUDriverPtr driver,
     VIR_FREE(priv->job.completed);
 
     cookie_flags = QEMU_MIGRATION_COOKIE_NETWORK |
-                   QEMU_MIGRATION_COOKIE_STATS;
+                   QEMU_MIGRATION_COOKIE_STATS |
+                   QEMU_MIGRATION_COOKIE_NBD;
     if (flags & VIR_MIGRATE_PERSIST_DEST)
         cookie_flags |= QEMU_MIGRATION_COOKIE_PERSISTENT;