xen: Setup Xen specific data for PVH

xen: Setup Xen specific data for PVH

Initialize the needed Xen specific data. This is:

- the Xen start of day page containing the console and Xenstore ring
  page PFN and event channel
- the grant table
- the shared info page

Write back the possibly modified memory map to the hypervisor in case
the guest is reading it from there again.

Set the RSDP address for the guest from the start_info page passed
as boot parameter.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Tested-by: Hans van Kranenburg <hans@knorrie.org>