Comment 4 for bug 1703423

Hi Michael,
You output confirms that you added like
  $ sudo add-apt-repository cloud-archive:ocata
at some point in the past.

This is what makes Ocata and dependencies available.

If you look at [1] you can see that there is only 2.5 in Xenial.
The last one according to your system being 1:2.5+dfsg-5ubuntu10.14

[1]: https://launchpad.net/ubuntu/+source/qemu