TL, DR. Disable AppArmor or SELinux, that will enables the volume to be mounted.
When using Cockpit to manage VMs, a problem occurs to me that it isn't able to attach a second volume. The owner group and user was also changed to
root:root instead of
I first tried to change the ownership manually using
chown, however, even if the ownership was changed, the problem still occurs, and no other solutions could be found on the internet, although some suggests to change the executer of virt-manager, but still got no luck.
After kept searching, I found another possible cause of the problem, the SELinux. Since I run kvm in a Ubuntu host, it inspired me to investigate the AppArmor.
This post suggest that the issue can be resolved by simply remove the AppArmor profile, I did so, and it works perfectly on the start. But then I discovered all the other VMs won't starting. Finally and desperately, I disabled the AppArmor for KVM.
The security will surely decrease and I'm sure it is not a proper solution for this problem. But I think this post may provides some insights for other amaeturs like me when having this problem occurs. And I might take some time to learn AppArmor for thinking about a better solution later on.