I accidently deleted the /etc/libvirt/qemu/networks/default.xml. How to restore it? Thanks.
On some distros you can find the original master in /usr/share/libvirt/networks. Alternatively it is in GIT upsteam.
Once you have the file, simply run as root:
virsh net-define default.xml
virsh net-start default
virsh net-autostart default # if you want it to run on boot