nested vmx: enable VMCS shadowing feature

nested vmx: enable VMCS shadowing feature

The current logic for handling the non-root VMREAD/VMWRITE is by
VM-Exit and emulate, which may bring certain overhead.

On new Intel platform, it introduces a new feature called VMCS
shadowing, where non-root VMREAD/VMWRITE will not trigger VM-Exit,
and the hardware will read/write the virtual VMCS instead.
This is proved to have performance improvement with the feature.

Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Acked-by: Jun Nakajima <jun.nakajima@intel.com>
Acked-by Eddie Dong <eddie.dong@intel.com>
Committed-by: Jan Beulich <jbeulich@suse.com>