On linux host servers, libvirtd uses dnsmasq to service the virtual networks, such as the default network. A new instance of dnsmasq is started for each virtual network, only accessible to guests in that specific network.
If you are running your own "global" dnsmasq, then this can cause your own dnsmasq to fail to start (or for libvirtd to fail to start its dnsmasq and the given virtual network). This happens because both instances of dnsmasq might try to bind to the same port number on the same network interfaces.
You have to change the global /etc/dnsmasq.conf as follows:
Either:
interface=eth0
or
listen-address=192.168.0.1
(Replace interface or listen-address with the interfaces or addresses you want your global dnsmasq to answer queries on).
And uncomment this line to tell dnsmasq to only bind specific interfaces, not try to bind all interfaces:
bind-interfaces