The environment variable $VIRSH_NET_CONFIG points to the default network configuration. Fortunately libvirt makes it very easy setup a NATted network bridge. Take a look at the corresponding configuration file _config/libvirt_nat_bridge.xml.
The network-description in this file tells libvirt to create a network bridge called nbr0. (This involves the configuration of iptables to act as NAT and to route IP-traffic. Furthermore it starts a dnsmasq process serving DHCP and DNS resolution for the virtual machines.)
The network bridge can be enabled and disabled with the commands network start|stop. Enabling means to deploy the configuration boot-persistent. network status displays the current setup. List all known host names with their associated IP-addresses with network lookup.
» vm network status
Name nat_bridge
UUID 0c3cf3f3-f6bc-ffdb-7ecc-32e89d2f59ff
Active: yes
Persistent: yes
Autostart: yes
Bridge: nbr0
» vm network lookup
10.1.1.2 lxdns01.devops.test
10.1.1.3 lxcm01.devops.test
10.1.1.4 lxrm01.devops.test
10.1.1.5 lxb001.devops.test
10.1.1.6 lxb002.devops.test
10.1.1.7 lxb003.devops.test
[...SNIP...]
On Ubuntu (>12.04) you can query the dnsmasq instance started from libvirt by adding its IP to the NetworkManager configuration. Write nameserver 10.1.1.1 into the file /etc/resolvconf/resolv.conf.d/head. After restarting the service with sudo restart network-manager you should be able to lookup hosts from the devops.test domain.