[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?