x86/PV: check GDT/LDT limits during emulation

x86/PV: check GDT/LDT limits during emulation

Accesses beyond the LDT limit originating from emulation would trigger
the ASSERT() in pv_map_ldt_shadow_page(). On production builds such
accesses would cause an attempt to promote the touched page (offset from
the present LDT base address) to a segment descriptor one. If this
happens to succeed, guest user mode would be able to elevate its
privileges to that of the guest kernel. This is particularly easy when
there's no LDT at all, in which case the LDT base stored internally to
Xen is simply zero.

Also adjust the ASSERT() that was triggering: It was off by one to
begin with, and for production builds we also better use
ASSERT_UNREACHABLE() instead with suitable recovery code afterwards.

This is XSA-298.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>