Hot resize of a virtio disk for a RHEL 5 guest in RHEV 3.3

RHEV 3.3 has added the ability to grow a virtual disk for the guest while the guest is still up. This works for RHEL 6 guests but doesn't work for RHEL 5 VMs. Resizing the disk in the RHEV-M web interface works as expected, but the new disk size isn't picked up by the guest OS.

The RHEL 5 guest is fully updated (latest kernel & rhevm-guest-agent packages) and I can hot add disks with no issues.

Is this a limitation of the RHEL 5 kernel? I can't find anything in the RHEV product documentation to suggest the resize disk functionality is not available for RHEL 5 guests.