xen/vcpu: Document the xen_vcpu_info and xen_vcpu

They are important structures and it is not clear at first
look what they are for.

The xen_vcpu is a pointer. By default it points to the shared_info
structure (at the CPU offset location). However if the
VCPUOP_register_vcpu_info hypercall is implemented we can make the
xen_vcpu pointer point to a per-CPU location.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[v1: Added comments from Ian Campbell]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>