x86: split __copy_{from,to}_user() into "guest" and "unsafe" variants

x86: split __copy_{from,to}_user() into "guest" and "unsafe" variants

The "guest" variants are intended to work with (potentially) fully guest
controlled addresses, while the "unsafe" variants are intended to be
used in order to access addresses not (directly) under guest control,
within Xen's part of virtual address space. Subsequently we will want
them to have distinct behavior, so as first step identify which one is
which. For now, both groups of constructs alias one another.

Double underscore prefixes are retained only on
__copy_{from,to}_guest_pv(), to allow still distinguishing them from
their "checking" counterparts once they also get renamed (to
copy_{from,to}_guest_pv()).

Add previously missing __user at some call sites.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org> [shadow]
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>