[ubuntu] 12.10 in kvm showns only black screen and big cursor

Hi - after searching the web for hours and updating three times, I thought it might be worth posting here, although I don't really have a very detailed error description:

I have an Ubuntu 12.04.1 host, running two 12.04.1 servers in VMs. Because of some issues, I thought I'd try upgrading a 12.04.1 (ubuntu server, no desktop) guest to 12.10. The upgrade runs fine, but after reboot I see Seabios and then a black screen with a BIG blinking cursor, 100% CPU load. Nothing else happens until I destroy the VM again.

Configured video hardware is Cirrus. All other options show no picture at all...

Any idea what I could do to get the 12.10 guest running?

Thanks!