x86/shadow: make sh_remove_write_access() helper HVM only

x86/shadow: make sh_remove_write_access() helper HVM only

Despite the inline attribute at least some clang versions warn about
trace_shadow_wrmap_bf() being unused in !HVM builds. Include the helper
in the #ifdef region.

Fixes: 8b8d011ad868 ("x86/shadow: the guess_wrmap() hook is needed for HVM only")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>