In any release of RHEL that has vhost-net enabled (evidenced by the presence of /dev/vhost-net, and the string "vhost=on" in the output of "/usr/libexec/qemu-kvm --help"), and that does not have this patch (and the corresponding patches for iptables (see Bug 612587) and the kernel (see Bug 605555) applied, some older guest OSes will be unable to acquire an IP address from the DHCP server started up by libvirt on the host (in particular, RHEL5 and older will have this problem). To work around this problem, you will need to disable vhost-net in the host, by preventing the vhost-net module from being loaded at host boot time.
To do this, create a file "/etc/modprobe.d/vhost-blacklist.conf" containing the following line:
install vhost_net /bin/true
and reboot the host. Don't forget to remove this file once the patched binaries are installed.