x86/segment: Bounds check accesses to emulation ctxt->seg_reg[]

x86/segment: Bounds check accesses to emulation ctxt->seg_reg[]

HVM HAP codepaths have space for all segment registers in the seg_reg[]
cache (with x86_seg_none still risking an array overrun), while the shadow
codepaths only have space for the user segments.

Range check the input segment of *_get_seg_reg() against the size of the array
used to cache the results, to avoid overruns in the case that the callers
don't filter their input suitably.

Subsume the is_x86_user_segment(seg) checks from the shadow code, which were
an incomplete attempt at range checking, and are now superceeded.  Make
hvm_get_seg_reg() static, as it is not used outside of shadow/common.c

No functional change, but far easier to reason that no overflow is possible.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>