I just upgrade a XS6.5 installation with XS7. All of my VM's are working except for one that is Ubuntu 12.04. Whenever I start it, the Xen portion of it works fine, but when the system attempts to boot from disk, it fails and the VM stops. No error messages...No logs....Just stops.
I fussed with the drive thinking that was the issue, but I thought why not create a new VM and mount the VDI to it.
To my surprise, I can't create a new Ubuntu 12.04 guest under XS7 either. Same failure. (VM "POSTS" but won't boot and fails to "stopped" state.)
You can boot either guest in "recovery mode" and they'll work fine.
So it would seem to me that something in the Ubuntu 12.x template isn't working too well with XS7.