Comment 4 for bug 928524

Here is the entry in the libvirt wiki describing this problem and the solution:

   http://wiki.libvirt.org/page/Virtual_network_%27default%27_has_not_been_started

In short, the systemwide dnsmasq instance should have "bind-interfaces" added to its configuration, and the particular interfaces you want it to use should be listed, otherwise dnsmasq will attempt to bind to all existing interfaces at the time it is started, and to any new interfaces that are created after that time.