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Normally dnsmasq will send a default route (the address of the host in the network definition) to any client requesting an address via DHCP. On an isolated network this makes no sense, as we have iptables to prevent any traffic going out via that interface, so anything sent that way would be dropped anyway. This extra/unusable default route becomes problematic if you have setup a guest with multiple network interfaces, with one connected to an isolated network and another that provides connectivity to the outside (example - one interface directly connecting to a physical interface via macvtap, with a second connected to an isolated network so that the host and guest can communicate (macvtap doesn't support guest<->host communication without an external switch that supports vepa, or reflecting all traffic back)). In this case, if the guest chooses the default route of the isolated network, the guest will not be able to get network traffic beyond the host. To prevent dnsmasq from sending a default route, you can tell it to send 0 bytes of data for the default route option (option number 3) with --dhcp-option=3 (normally the data to send for the option would follow the option number; no extra data means "don't send this option"). I have checked on RHEL5 (a good representative of the oldest supported libvirt platforms) and its version of dnsmasq (2.45) does support --dhcp-option, so this shouldn't create any compatibility problems.
libvirt library code README
===========================
The directory provides the bulk of the libvirt codebase. Everything
except for the libvirtd daemon and client tools. The build uses a
large number of libtool convenience libraries - one for each child
directory, and then links them together for the final libvirt.so,
although some bits get linked directly to libvirtd daemon instead.
The files directly in this directory are supporting the public API
entry points & data structures.
There are two core shared modules to be aware of:
* util/ - a collection of shared APIs that can be used by any
code. This directory is always in the include path
for all things built
* conf/ - APIs for parsing / manipulating all the official XML
files used by the public API. This directory is only
in the include path for driver implementation modules
* vmx/ - VMware VMX config handling (used by esx/ and vmware/)
Then there are the hypervisor implementations:
* esx/ - VMware ESX and GSX support using vSphere API over SOAP
* lxc/ - Linux Native Containers
* opennebula/ - Open Nebula using XMLRPC
* openvz/ - OpenVZ containers using cli tools
* phyp/ - IBM Power Hypervisor using CLI tools over SSH
* qemu/ - QEMU / KVM using qemu CLI/monitor
* remote/ - Generic libvirt native RPC client
* test/ - A "mock" driver for testing
* uml/ - User Mode Linux
* vbox/ - Virtual Box using native API
* vmware/ - VMware Workstation and Player using the vmrun tool
* xen/ - Xen using hypercalls, XenD SEXPR & XenStore
* xenapi/ - Xen using libxenserver
Finally some secondary drivers that are shared for several HVs.
Currently these are used by LXC, OpenVZ, QEMU, UML and Xen drivers.
The ESX, OpenNebula, Power Hypervisor, Remote, Test & VirtualBox
drivers all implement the secondary drivers directly
* cpu/ - CPU feature management
* interface/ - Host network interface management
* network/ - Virtual NAT networking
* nwfilter/ - Network traffic filtering rules
* node_device/ - Host device enumeration
* secret/ - Secret management
* security/ - Mandatory access control drivers
* storage/ - Storage management drivers
Since both the hypervisor and secondary drivers can be built as
dlopen()able modules, it is *FORBIDDEN* to have build dependencies
between these directories. Drivers are only allowed to depend on
the public API, and the internal APIs in the util/ and conf/
directories