x86: refine address validity checks before accessing page tables

x86: refine address validity checks before accessing page tables

In commit 40d66baa ("x86: correct LDT checks") and d06a0d71 ("x86: add
address validity check to guest_map_l1e()") I didn't really pay
attention to the fact that these checks would better be done before the
paging_mode_translate() ones, as there's also no equivalent check down
the shadow code paths involved here (at least not up to the first use
of the address), and such generic checks shouldn't really be done by
particular backend functions anyway.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>