I use puppet to keep my machines in sync with each other. Our default configuration has haldaemon and messagebus (dbus) off. I ran into a problem when I tried to create a new vm and there was no option to use the shared phyical device (the bridge)
In order for virt-manager to see the bridge, I needed to have haldaemon and messagebus turned on. Silly problem, but I couldn't find the answer without using strace myself...hope that helps someone.