You could also set up an ISO library in XenCenter using an existing NFS or CIFS share where you can store the ISO images you want to install from. If you don't have any NFS share yet, I would recommend creating a fileserver VM using the TurnKey Linux Fileserver appliance.
This way you don't need to mount the net install ISO, you can simply "insert" it into the VM's optical drive and boot the VM from it. However, if the net install ISO doesn't contain the proper kernel for XenServer, than you should use the "Other" template instead of a "CentOS" template when creating the VM in XenCenter, and convert it into a PV guest after the installation is complete, since you'll get a HVM guest this way. You can find my instructions for that here.