Change I8a705114d47384fcd00955d4a4f204072fed57c2 (written by me... sigh) addressed a bug which prevented live migration to a target host with overcommitted disk when made with microversion <2.25. It achieved this, but the fix is still not correct. We now do:

        if disk_over_commit:
            disk_available_gb = dst_compute_info['local_gb']

Unfortunately local_gb is *total* disk, not available disk. We actually want free_disk_gb. Fun fact: due to the way we calculate this for filesystems, without taking into account reserved space, this can also be negative.

The test we're currently running is: could we fit this guest's allocated disks on the target if the target disk was empty. This is at least better than it was before, as we don't spuriously fail early. In fact, we're effectively disabling a test which is disabled for microversion >=2.25 anyway. IOW we should fix it, but it's probably not a high priority.