xen: Use a typesafe to define INVALID_GFN

xen: Use a typesafe to define INVALID_GFN

Also take the opportunity to convert arch/x86/debug.c to the typesafe gfn.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Kevin Tian <kevin.tian@intel.com>