We somehow regressed the vcpu hotplugging between -105 and -115, need to complete the bisection to see what happened.
Miroslav said he couldn't reproduce this problem, so I tried again and also cannot. Unfortunately I don't have any logs from the time the problem was occurring. It's possible my xend was in a strange state due to working on another bug and simply restarting it would have been sufficient (rather than the full downgrade). We can close this as NOTABUG.