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>