I posted this to libvirt-users at https://www.redhat.com/archives/libvirt-users/2012-September/thread.html#00095
$ sudo apt-get install libvirt-bin
$ BRIDGE="mybr0"; IP="192.168.123.1";
$ cat > $BRIDGE.xml <<EOF
<network>
<name>$BRIDGE</name>
<forward mode='nat'/>
<bridge name='$BRIDGE' stp='off' delay='0' />
<ip address='$IP' netmask='255.255.255.0'>
</ip>
</network>
EOF
$ sudo virsh -c qemu:///system net-define $BRIDGE.xml
$ sudo virsh -c qemu:///system net-start $BRIDGE
$ ps axw | grep dnsm | grep $BRIDGE.pid
9888 ? S 0:00 /usr/sbin/dnsmasq -u libvirt-dnsmasq --strict-order --bind-interfaces --pid-file=/var/run/libvirt/network/mybr0.pid --conf-file= --except-interface lo --listen-address 192.168.123.1
$ dig +short @$IP www.ubuntu.com
91.189.90.41
Above shows that the given network I did not request any dns server to be run, yet libvirt is running one for me.
ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: libvirt-bin 0.9.13-0ubuntu10
ProcVersionSignature: User Name 3.5.0-15.20-generic 3.5.4
Uname: Linux 3.5.0-15-generic x86_64
ApportVersion: 2.5.2-0ubuntu2
Architecture: amd64
Date: Thu Sep 20 13:38:38 2012
Ec2AMI: ami-e3df6c8a
Ec2AMIManifest: (unknown)
Ec2AvailabilityZone: us-east-1b
Ec2InstanceType: t1.micro
Ec2Kernel: aki-825ea7eb
Ec2Ramdisk: unavailable
ProcEnviron:
TERM=screen
PATH=(custom, no user)
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: libvirt
UpgradeStatus: No upgrade log present (probably fresh install)
Just for the record, I realize i could do this in any other number of ways than libvirt. However, the description easily brings up a network in some human understandable fashion, fails if that network already exists, and easily starts it automatically for me on reboot. Doing that combination of things just about any other way requires editing files in /etc/network or adding upstart or sysvinit jobs.