libxl: fix vm lock overwritten bug

libxl: fix vm lock overwritten bug

In libxl driver we do virObjectRef in libxlDomainObjBeginJob,
If virCondWaitUntil failed, it goes to error, do virObjectUnref,
There's a chance that someone undefine the vm at the same time,
and refs unref to zero, vm is freed in libxlDomainObjBeginJob.
But the vm outside function is not Null, we do virObjectUnlock(vm).
That's how we overwrite the vm memory after it's freed. I fix it.

Signed-off-by: Wang Yufei <james.wangyufei@huawei.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>