libxl: always enable pae for x86_64 HVM
    
    For HVM domains, pae is only set in libxl_domain_build_info when
    explicitly specified in the hypervisor <features> config. This is
    fine for i686 machines, but is incorrect behavior for x86_64 machines
    where pae must always be enabled. See the following discussion for
    additional details
    
    https://www.redhat.com/archives/libvir-list/2017-January/msg00254.html