libxl: set nestedhvm for mode host-passthrough

libxl: set nestedhvm for mode host-passthrough

Xen feature nestedhvm is the option on Xen 4.4+ which enables
nested virtualization when mode host-passthrough is applied.

nested HVM is enabled by adding below on the target domain;
<cpu mode='host-passthrough'/>

Virtualization on target domain can be disabled by specifying
such under feature policy rule on target name;

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

or:

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

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Wim ten Have <wim.ten.have@oracle.com>