Had the same issue. The solution is actually really easy.
If you follow this tutorial http://www.howtoforge.com/virtualiza...virtualization
you are adding an additional repository from http://au1.mirror.crc.id.au/repo/
and there is xen 4.1.3 available. libvirt is incompatible to xen 4.2 at the moment so you have to use 4.1.3.
After you added the repository just do
yum install xen-4.1.3-2.el6.x86_64 xen-devel-4.1.3-2.el6.x86_64
thats it. now you should be able to patch libvirt just fine. You need to make sure that if you used yum install xen that you remove all references to xen-4.2 or you will get lots of errors. So uninstall everything for xen 4.2 before you install xen-4.1.3