libs/guest: Reduce number of iovecs in write_split_record()

Construct xc_sr_rhdr directly rather than in two halves.

Use size_t for combined_len, which allows the later size check to spot a few
more cases which might have truncated.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>

tools/libs/guest/xg_sr_common.c

@@ -59,11 +59,11 @@ int write_split_record(struct xc_sr_context *ctx, struct xc_sr_record *rec,
     static const char zeroes[REC_ALIGN] = {};
 
     xc_interface *xch = ctx->xch;
-    typeof(rec->length) combined_length = rec->length + sz;
+    size_t combined_length = rec->length + sz;
     size_t record_length = ROUNDUP(combined_length, REC_ALIGN);
+    struct xc_sr_rhdr rhdr = { rec->type, combined_length };
     struct iovec parts[] = {
-        { &rec->type,       sizeof(rec->type) },
-        { &combined_length, sizeof(combined_length) },
+        { &rhdr,            sizeof(rhdr) },
         { rec->data,        rec->length },
         { buf,              sz },
         { (void *)zeroes,   record_length - combined_length },