[6.9.0] VM Edit to Add Mount Tags Hangs Forever

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If I try to edit an existing VM to add a mount tag, it hangs forever. 

I can edit the XML without issue. I don't know enough about the XML settings to try and add the second mount tag manually. 

If I create a new VM, I can add as many mount tags as I want and it works fine. 

I first noticed the issue in 6.8.3.

tower-diagnostics-20210301-2231.zip