Per xen-xl conversions from and to native under host-passthrough
mode we take care for Xen (nestedhvm = mode) applied and inherited
settings generating or processing correct feature policy:

[On Intel (VT-x) architectures]
<feature policy='disable' name='vmx'/>

or

[On AMD (AMD-V) architectures]
<feature policy='disable' name='svm'/>

It will then generate (or parse) for nestedhvm=1 in/from xl format.
Signed-off-by: NJoao Martins <joao.m.martins@oracle.com>
Signed-off-by: NWim ten Have <wim.ten.have@oracle.com>