libxl: Remove unnecessary buffer zeroing and zalloc()

When reading the console, xen overwrites the contents of the buffer,
so there is no need to zero the buffer before passing it to xen.
Instead, add a NULL at the end of the buffer.

While we are at it, change the zalloc() of the buffer back to
malloc() as it was before bdf4131 (libxl: don't leak buf in
libxl_xen_console_read_start error handling, 2013-12-03).  The comment
in that commit message says that the intent of the commit was to
change malloc+memset to zalloc(), but only for the
libxl_xen_console_reader struct, not for the buffer.

Signed-off-by: Javi Merino <javi.merino@cloud.com>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>

tools/libs/light/libxl_console.c

@@ -783,7 +783,7 @@ libxl_xen_console_reader *
     unsigned int size = 16384 + 1;
 
     cr = libxl__zalloc(NOGC, sizeof(libxl_xen_console_reader));
-    cr->buffer = libxl__zalloc(NOGC, size);
+    cr->buffer = libxl__malloc(NOGC, size);
     cr->size = size;
     cr->clear = clear;
     cr->incremental = 1;
@@ -813,7 +813,6 @@ int libxl_xen_console_read_line(libxl_ctx *ctx,
     unsigned int nr_chars = cr->size - 1;
     GC_INIT(ctx);
 
-    memset(cr->buffer, 0, cr->size);
     ret = xc_readconsolering(ctx->xch, cr->buffer, &nr_chars,
                              cr->clear, cr->incremental, &cr->index);
     if (ret < 0) {
@@ -823,6 +822,7 @@ int libxl_xen_console_read_line(libxl_ctx *ctx,
     }
     if (!ret) {
         if (nr_chars) {
+            cr->buffer[nr_chars] = '\0';
             *line_r = cr->buffer;
             ret = 1;
         } else {