Tried all of the above suggestions and more with out success.
Created a new VM on the same host (CentOS 6.9) using virt-manager and that worked.
The not working VM definition had
<os><type arch="x86_64" machine="rhel5.4.0">hvm</type></os>
Changed this to
<os><type arch="x86_64" machine="rhel6.6.0">hvm</type></os>
based on the working VM using virsh edit and it now works.
There was also a difference in the "feature policy" loaded (invtsc was missing), which I reloaded using virt-manager.