Just installed a fresh Ubuntu 13.04 going up from 12.04, similar kvm-qemu and bridge setup, perhaps. The VMs are on raw VG-LVM images. I will watch out for a panic. Here's the bridge setup (that worked reliably in 12.04) and was copied over to 13.04, so far so good...
12.04 may be more stable than 13.04, but in this case there are some features I wanted to try out, namely Spice, which seem to be improved in 13.04
~$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
iface lo inet loopback
# The primary network interface
iface p4p1 inet manual
iface br0 inet static
up ip addr add 192.168.133.2/24 brd 192.168.133.255 dev br0 label br0:0
dns-nameservers 22.214.171.124 126.96.36.199
I have separated the LVM's into individual Volume Groups: a 1 TB disk contains /boot and RAID1-swap, plus RAID-1 MD0 and MD1 for system volume groups: VG0 contains LV's for / and /home, VG1 contains LVs for the VM disk images. This way I can run the normal installer from a thumb and use the stock partitioner to reset /, /boot, swap while leaving /home and the VM images untouched. Then it's not so hard to change distros.