Changing the mac address in a libvirt xml config file breaks network connectivity for the guest

You don't need to create the bridge network in libvirt. You create the bridge using the OS tools (bridge-utils for RHEL).

Here is how you can do that in RHEL/Fedora.

Then, when you create the VM, tell virt-install (or virt-manager) to use the bridge you just created.

IMO, if you need to use XEN, use Citrix XenServer, as it has tools that make things easier for managing multiple VMs. Otherwise use KVM, it is going to be the better supported option from Redhat.

The answer to this issue was that because I'd done a non-graphic install of Centos 6 minimal on the guests, the network interfaces had not been automatically set up to work. My lack of knowledge of virtualisation and Centos meant that I assumed that this would be taken care of for me, but I was incorrect in this assumption.

Fix: I needed to go into ifcfg-eth0 on the guests and turn on ONBOOT=yes and BOOTPROTO=dhcp, then restart the network adapter via ifdown eth0 then ifup eth0. After doing this, the guests received IP addresses and everything worked as expected.