It might be that the host is not reporting the host capabilities correctly. You can see all host's supported CPUs in the host detail screen, just roll over the info icon next to the CPU Type.
You can check if the data are in the database like this:
select cpu_flags from vds where vds_name = 'your-host-name';
And you can check what the host reports by running the following command on the host (search for cpuFlags key):
sudo vdsm-client Host getCapabilities
Make sure the capabilities contain all required flags - vmx, model_Haswell-noTSX and nx.
Regards,
Lucia