[CentOS] libvirt machine type rhel6.6.0 on CentOS 7

One of our VM clusters has four CentOS 6.6 machines. I'm in the 
process of testing the addition of a CentOS 7 box (with the goal of 
upgrading the 6.6 machines soon).

A newly created VM running on a 6.6 node has a machine type of 
rhel6.6.0:

   <os>
     <type arch='x86_64' machine='rhel6.6.0'>hvm</type>
     <boot dev='hd'/>
   </os>

The libvirt on the CentOS 7 machine won't recognize that label; it 
goes straight from rhel6.5.0 to the newer RHEL 7 types: 
pc-i440fx-rhel7.0.0 and pc-q35-rhel7.0.0.

I can shutdown the VM, edit its XML definition, restart it, and then 
migrate it -- but I'd rather be able to do a live migration without a 
shutdown.

Has anyone else seen this? Any workarounds?

-- 
Paul Heinlein
heinlein at madboa.com
45°38' N, 122°6' W