We have twice previously attempted to remove Xenner
support

  commit de9be0ab
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Wed Aug 22 17:29:01 2012 +0100

    Remove xenner support

  commit 92572c3d
  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: NDaniel P. Berrange <berrange@redhat.com>