x86/xen: globalize have_vcpu_info_placement

have_vcpu_info_placement applies to both PV and HVM and as we're going
to split the code we need to make it global.

Rename to xen_have_vcpu_info_placement.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>