I recently ran into an issue after upgrading my XenDesktop environment to 7.12 from 7.11.  Prior to that update I was able to create MCS desktops in my XenServer pool with no issues.  After upgrading to 7.12 the newly created VMs would not register with the delivery controller, and according to XenServer they did not have the tools installed.  I found that the network card didn’t even appear in Network Connections, and in device manager it showed up as a Realtek adapter instead of the XenServer PV device it should have.

After further investigation I found that the new VMs were created missing some things in XenServer if compared to VMs which were working (xe vm-list name-label=<name of vm> params=all).  I posted on discussions.citrix.com to see if anyone had run into this issue before, and Carl Stalhood pointed me to this post.

Apparently, there is a private hotfix that will correct the problem, but it requries you do delete/re-create the machine catalog (which shows as the answer to the problem on the thread).  Reading through the post Paul Browne gives a much better/easier solution:
“Also in the short term to get your VM`s working correctly.

Shutdown the VM
putty or console into the XenServer

xe vm-list

(find UUID of the Vm you wish to fix)

xe vm-param-set has-vendor-device=true uuid=<UUID of the VM>

It should now work as expected.”

I wrote this PowerShell script to perform the operation, that Paul gives above, automatically across multiple VMs.  Requires Powershell v3, scripts enabled (set-executionpolicy to change it), and XenCenter to be installed on the machine the script is run from.  Read the comments of the script to see an explanation of what the script is doing.