xen: change the sizes of fields in the HVM start info layout to be 64bits

xen: change the sizes of fields in the HVM start info layout to be 64bits

At the moment the only consumer of this structure is x86, but other arches
might also use it, so make all the fields 64bits. On x86 Xen will still try
to place everything below the 4GiB boundary, but that might not be feasible
in other arches.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Requested-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>