tools/blktap2: fix hypothetical buffer overflow

tools/blktap2: fix hypothetical buffer overflow

gcc-8 complains:

    vhd-util-read.c: In function 'vhd_util_read':
    vhd-util-read.c:50:24: error: '%lu' directive output may be truncated writing between 1 and 20 bytes into a region of size 15 [-Werror=format-truncation=]
      snprintf(nbuf, nsize, "%" PRIu64, num);
                            ^~~
    vhd-util-read.c:50:25: note: format string is defined here
      snprintf(nbuf, nsize, "%" PRIu64, num);
    vhd-util-read.c:50:24: note: directive argument in the range [0, 18446744073709551614]
      snprintf(nbuf, nsize, "%" PRIu64, num);
                            ^~~
    vhd-util-read.c:50:2: note: 'snprintf' output between 2 and 21 bytes into a destination of size 15
      snprintf(nbuf, nsize, "%" PRIu64, num);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    vhd-util-read.c:43:24: error: '%#lx' directive output may be truncated writing between 1 and 18 bytes into a region of size 15 [-Werror=format-truncation=]
      snprintf(nbuf, nsize, "%#" PRIx64 , num);
                            ^~~~
    vhd-util-read.c:43:25: note: format string is defined here
      snprintf(nbuf, nsize, "%#" PRIx64 , num);
    vhd-util-read.c:43:24: note: directive argument in the range [0, 18446744073709551614]
      snprintf(nbuf, nsize, "%#" PRIx64 , num);
                            ^~~~
    vhd-util-read.c:43:2: note: 'snprintf' output between 2 and 19 bytes into a destination of size 15
      snprintf(nbuf, nsize, "%#" PRIx64 , num);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Make the buffer larger.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Juergen Gross <jgross@suse.com>