It will cause that create vm failed When manager
tool is killed forcibly (kill -9 libvirtd_pid),
the file not was unlink, and unlock. It's better
that report the error message for users.

Signed-off-by: Huangweidong <weidong.huang@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>