xen: introduce XEN_GUEST_HANDLE_PARAM

xen: introduce XEN_GUEST_HANDLE_PARAM

XEN_GUEST_HANDLE_PARAM is going to be used to distinguish guest pointers
stored in memory from guest pointers as hypercall parameters.

guest_handle_* macros default to XEN_GUEST_HANDLE_PARAM as return type.
Two new guest_handle_to_param and guest_handle_from_param macros are
introduced to do conversions.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>