x86emul: constify write_segment() register pointer

Since I stumbled across this while looking for further constification
opportunities, also correct the insn_fetch() related comment.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I would have wanted to also constify the pointers passed to .write(),
.cmpxchg(), and .rep_stos(), but that doesn't work (not only) because
of hvmemul_do_mmio_buffer() being used for both directions.

--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -1439,7 +1439,7 @@ static int hvmemul_read_segment(
 
 static int hvmemul_write_segment(
     enum x86_segment seg,
-    struct segment_register *reg,
+    const struct segment_register *reg,
     struct x86_emulate_ctxt *ctxt)
 {
     struct hvm_emulate_ctxt *hvmemul_ctxt =
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -200,7 +200,10 @@ struct x86_emulate_ops
 
     /*
      * insn_fetch: Emulate fetch from instruction byte stream.
-     *  Parameters are same as for 'read'. @seg is always x86_seg_cs.
+     *  Except for @bytes parameters are same as for 'read'.
+     *  @bytes: Access length (0 <= @bytes < 16, with zero meaning
+     *  "validate address only").
+     *  @seg is always x86_seg_cs.
      */
     int (*insn_fetch)(
         enum x86_segment seg,
@@ -306,7 +309,7 @@ struct x86_emulate_ops
      */
     int (*write_segment)(
         enum x86_segment seg,
-        struct segment_register *reg,
+        const struct segment_register *reg,
         struct x86_emulate_ctxt *ctxt);
 
     /*