Fix locking with qemuMigrationPrepareTunnel method

The qemuMigrationPrepareTunnel method should not unlock the
qemu driver, since that is the caller's job.

* src/qemu/qemu_migration.c: Fix qemuMigrationPrepareTunnel
  unlocking of QEMU driver

src/qemu/qemu_migration.c

@@ -369,7 +369,6 @@ cleanup:
         virDomainObjUnlock(vm);
     if (event)
         qemuDomainEventQueue(driver, event);
-    qemuDriverUnlock(driver);
     return ret;
 }