You can’t add extra Core to a CPU socket though XenCenter, you need to start you console of the XenServer.

To Add 4 Core in each CPU Socket you need to run this commands:

xe vm-param-set platform:cores-per-socket=4 uuid=<VM UUID>

To found the vm’s UUID, you can use these commands:

xe vm-list name-label=<Name of the VM>

After you have put 4 core in each CPU socket you also need to run these commands:

xe vm-param-set VCPUs-max=8 uuid=<VM UUID>
xe vm-param-set VCPUs-at-startup=8 uuid=<VM UUID>

This will add 8 CPU in totalt, but only 2 CPU socket with each 4 Core.