x86/xen: use xen_vcpu_id mapping when pointing vcpu_info to shared_info

shared_info page has space for 32 vcpu info slots for first 32 vCPUs
but these are the first 32 vCPUs from Xen's perspective and we should
map them accordingly with the newly introduced xen_vcpu_id mapping.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>