block/sheepdog: Use QEMU_NONSTRING for non NUL-terminated arrays

GCC 8 added a -Wstringop-truncation warning:

  The -Wstringop-truncation warning added in GCC 8.0 via r254630 for
  bug 81117 is specifically intended to highlight likely unintended
  uses of the strncpy function that truncate the terminating NUL
  character from the source string.

This new warning leads to compilation failures:

    CC      block/sheepdog.o
  qemu/block/sheepdog.c: In function 'find_vdi_name':
  qemu/block/sheepdog.c:1239:5: error: 'strncpy' specified bound 256 equals destination size [-Werror=stringop-truncation]
       strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  make: *** [qemu/rules.mak:69: block/sheepdog.o] Error 1

As described previous to the strncpy() calls, the use of strncpy() is
correct here:

    /* This pair of strncpy calls ensures that the buffer is zero-filled,
     * which is desirable since we'll soon be sending those bytes, and
     * don't want the send_req to read uninitialized data.
     */
    strncpy(buf, filename, SD_MAX_VDI_LEN);
    strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);

Use the QEMU_NONSTRING attribute, since this array is intended to store
character arrays that do not necessarily contain a terminating NUL.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>