qemu: really remove last traces of Xenner support
    
    We have twice previously attempted to remove Xenner
    support
    
      commit de9be0ab4d7abe4c3ae5beaaff7ddb208921f1f1
      Author: Daniel P. Berrange <berrange@redhat.com>
      Date:   Wed Aug 22 17:29:01 2012 +0100
    
        Remove xenner support
    
      commit 92572c3d71bd08e6dd1897c7b14be9d27df608e9
      Author: Ján Tomko <jtomko@redhat.com>
      Date:   Wed Feb 18 16:33:50 2015 +0100
    
        Remove code handling the QEMU_CAPS_DOMID capability
    
    This change really does remove the last traces of it
    in the capabilities handling code
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>