Comment 4 for bug 1641532

I'm afraid there is no "perfect" fix for it - so I need your help evaluating.
And as I assumed it is worse than it seemed at first.

The reason for your finding "I have seen that in 2.3, the utopic machine is a kvm-2.3 machine while it's a kvm-2.5 machine is 2.5 when it should be kvm-2.3 too" is the one being complex.
I assume you likely checked that in the past it was 2.3 on a Vivid host where qemu was on 2.3.

The problem is that the definition always semed to just inherit "latest".
So on Xenial being 2.5 it got "2.5".
On Vivid where it was 2.3 it got 2.3.
IMHO that is wrong back to when it started - utopic actually was "2.1" so that is what it should be (and always have been).

Your fix to make it a 2.3 therefore is only correct for your case where your guest was "last started" on a qemu 2.3 if I'm not mistaken.
It should truly be a 2.1 type.

The worst in all of this is this: If you happened to create that guest back in a day it was 2.1.
But if in the meantime you restarted the guest on let's say the qemu 2.3 Host then it "became" a 2.3 guest (which is wrong). The same is true to the even more relevant trusty type.
But everybody out there might have restarted on different types - so I have to assume that I can find Trusty types on any of 2.0 - 2.5 out there (the same applies to utopic types).

So if we now make it "correctly" a 2.1 type we might still not help you and on top break others :-/
But if we make it a 2.3 type we might only help you and others in the same case as you.
But at the same time we break all others :-/

TL;DR - do fix for those that never restarted; or if not which release of the "restarters" do we fix :-/++

Still thinking ....