Here's what I get when I patch:
# patch -p0 < ~/src/libvirt-spec-rhel6-enable-xen.patch
patching file libvirt.spec
Hunk #1 succeeded at 137 with fuzz 1 (offset 33 lines).
I did notice that when installing libvirt-0.9.10-21.el6.src.rpm the howto mentions that we would get warnings we could ignore. I got a screenful of the warnings about mockbuild not existing - enough to fill my scrollback buffer. Would either of those make any difference?