Using the 'require' value for the 'policy' attribute indicates
that the guest will have the feature so the host CPU does not
need to support it if the hypervisor can emulate it.

E.g. 'x2apic' is emulated by QEMU even if the host does not support it:

    <feature policy='require' name='x2apic'/>
Signed-off-by: NKashyap Chamarthy <kchamart@redhat.com>