x86/pv: Adjust the save_segments() comment regarding MSR_GS_SHADOW

This is slightly stale mentioning SWAPGS and not LKGS.  However, take the
opportunity to make the comment more general and less likely to bitrot.

It would be a serious vulnerability for operating systems generally if CPL3
could modify GS_SHADOW at all.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Teddy Astie <teddy.astie@vates.tech>

xen/arch/x86/domain.c

@@ -1951,9 +1951,8 @@ static void load_segments(struct vcpu *n)
  * changes to bases can also be made with the WR{FS,GS}BASE instructions, when
  * enabled.
  *
- * Guests however cannot use SWAPGS, so there is no mechanism to modify the
- * inactive GS base behind Xen's back.  Therefore, Xen's copy of the inactive
- * GS base is still accurate, and doesn't need reading back from hardware.
+ * Guests cannot modify the inactive GS base behind Xen's back.  Therefore
+ * Xen's copy is still accurate and doesn't need reading back.
  *
  * Under FRED, hardware automatically swaps GS for us, so SHADOW_GS is the
  * active GS from the guest's point of view.