You need qemu-spice for using SPICE, which does not ship with RHEL5 or RHEL6. On top of that, SPICE is only supported by Red Hat for RHEV, not libvirt. That may change in the future, ... but when, nobody knows ;-)
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No you don't Dag.
qemu-kvm and libvirt in RHEL6 already supports SPICE... the only thing that isn't included is support for it in virt-manager (that is coming down the road) but you can enable it with virsh edit easily enough following the XML definition at the libvirt fine.
I was playing with it last week - very impressive piece of technology.
James